I am 72 and still go on a lot of long hikes and climbs so I understand the weight concern. You don't mention what you are using so it's hard to give specific advice. Two years ago, I bought a Canon RP plus RF 24-105 4L specifically for light weight. The combo weighs 45 ounces. I took this on a climb last year requiring 12 miles and 5600 ft. of gain at altitude. However, my older EF 24-70 2.8L II takes noticeably better images at every aperture setting but adds 1/2 lb. Depending on what I expect to see, I'll sometimes take a RF 70-200 4L IS instead. It weighs about 1 oz more than the 24-105 but has better IQ. Another possibility is to use light weight primes instead. Sometimes, I took a 24 2.8 plus 50 1.8 with me instead of a zoom. The pair weighed about a pound. However, I still use my R5 and the best lens(es) I have for everything other than strenuous outings.

I preordered the 135 1.8 November 3. I got it yesterday!

A friend of mine drove a Honda Fit and had 3 Ferraris at the same time for weekends!

The 5D4 is certainly one of the best DSLRs ever made. It is unlikely I would have bought a 5D5 had Canon gone that way. But 18 months ago, I bought a R5.

It makes the 5D4 seem like an antiquated lump.

Here's why:
The AF system is no doubt the main feature. But it's hard to explain just how much of a game changer it is: Faster, more precise, tracking, eye detect. I let a friend who uses Sony try it out on birds. He called it cheating! Eye and face tracking allows you to instantly compose and recompose your shots. No more messing with changing your focus point or back button focus. When using longer lenses, the number of "near misses" are greatly reduced, if not eliminated.

Much higher shutter speeds and a buffer that you are unlikely to ever fill up. I was shooting a flying flock of Ibis once and got about 90 shots in the sequence. The buffer never slowed me and there were only three shots that weren't very precisely focused. With a 5D4, I'd be lucky to get 20 shots out of it and, if I was really lucky, 10 that were focused precisely enough.

IQ: I wasn't expecting much improvement but, after a while, I began to sense better color and images seemed more 3 dimensional for lack of a better word. One feature that gets little attention, however, is the big improvement in dynamic range. Sony always had bragging rights on this but the R5 closed the gap. This is really noticeable in challenging light conditions. I'm getting really nice shots that would have been impossible with the 5D4. This has motivated me to experiment with challenging lighting to produce compelling images.

MP: I'd be perfectly happy if it "only" had 30MP. We've gotten to diminishing returns in terms of increasing MP. I have a 300 2.8L IS II which is allegedly Canon's sharpest lens. Under ideal conditions, I can crop a little more but resolution is more likely to be capped by atmospheric conditions or lens quality. So anyone expecting their images will suddenly be much sharper because of 45MP will be disappointed. The biggest effect is much bigger files.

It is about 6 oz. lighter than a 5D4 though this benefit is largely negated when using a ring adapter.

All of your EF lenses will perform better to varying degrees with a R5. I've only replaced a few of my 12 EF lenses with RF versions. Noteworthy in my mind is the 100-500. It's a lot lighter than the 100-400L II and performs better in every way.

It is unlikely I'll buy a R5 II whenever it appears. If more MP is the main attraction, I'll definitely pass.

One more thing. If you intend to sell your 5D4 after buying a R5, the value of the 5D4 keeps falling and will likely continue to do so.

Hope this helps.

Like you, I am old (72) and still go on long, strenuous hikes and climbs. I own a EF 35 2.0 IS and 1.4L II for exactly the same reason you're considering the 85 1.2L. I bought the 1.4 used as well. I don't use either lens a whole lot and sometimes think I should have just bought one. But I expect both will be in my estate so I'm OK with that. I've had a 85 1.8 for many years and don't use it a whole lot so I don't have any specific advice on that focal length. I know the 85 1.2 has been knocked for slow and sometimes not accurate AF but I'll bet it performs a lot better on a R5 vs 5D4. Used ones seem like good bang for the buck.

Bottom line: I would wait a bit and assess just how much an 85 1.2 can do something your 85 2.0 or 135 2L cannot. Make sure it is the best use of 1000. The main reason I don't use my 85 much is I'd rather use the 135 which creates magical images. BTW, the 135 has become quite a legend with legions of passionate users, including me. I don't see the same level of passion for the 85. I have preordered the RF 135 1.8. I guess millions of others have as well because I'm still waiting!

Someday, someone will make a 14-600mm f/1.2 lens that weighs 2 lbs., is sharper than any prime with better contrast, better bokeh, better AF and less distortion than the best prime. Then this argument will be moot. Sadly, I'll be dead.

I agree that zooms have come a long way in the last ten years or so. I own a few of Canon's finest and use them a lot. The EF 24-70 2.8L II has been my workhorse for years. Lately, I've been going through older images to clear hard drive space and I noticed that a majority of the images that really grab me were taken with primes. There is an "it" factor I can't really define. Clearly, resolution is not the only factor, maybe not even the main factor. I know I look forward to using primes because I expect some special images. In deference to Rick from NY, I don't know how many lay people would really see the difference, especially in prints. But they may sense it anyway.

I have a 135 2L also and I figure it negates the need for the 2.8 for me. If I were a pro wedding photographer, I'd have a 70-200 2.8 but it would probably be in addition to a f/4. The R5 definitely improves the AF on the 135 and the IBIS helps too. I have preordered the RF 135 1.8L IS. It should be even more useful for me. It will also be my first RF L prime. Interestingly, Vanessa Joy, who does truly beautiful portraiture and wedding work, calls the 135 her "secret weapon."

The 70-200 is a joy to take hiking. It's only about 1.5 ounces lighter (8 ounces including the adapter) than the EF version but the compact size makes all the difference when it's slung over my shoulder for long distances. Sometimes I throw a RF 50 1.8 in the bag if I think I'll need a wider angle. For more strenuous hikes or climbs, I use it with a RP instead of the R5. The combo is less than 3 lbs.

Canon has been criticized before for not providing hods with its non L lenses, especially those costing 500. or more. Vello has nicely made "knock off" hoods for about 1/3 Canon's price if you want one. I would recommend using one on the RF 100-400.

If you shoot anything that moves, you should be ordering a R5 right now.

I went from a 5D4 to a R5 18 months ago. The biggest benefit by far is the greatly improved AF, eye detect and tracking. The additional megapixels are really a small benefit. I shoot a lot of birds and, yes, I can crop a little more as long as the image is really sharp to begin with. However, image sharpness is often limited by the lens, atmospheric conditions or both. Don't expect miracles. All of your EF lenses will perform better on a R5, some more than others. As far as IQ goes, don't expect a lot. I think the color is better and overall images have a better ambiance, but it is subtle. The 5D4 is already really good. Noise control is a lot better. Often overlooked, however, is the greatly improved dynamic range which will allow you to get better images in challenging lighting conditions. Sony had a big lead in this dept. but the R5 closed the gap. After I used the R5 for a bit, the 5D4 seemed like an outdated lump. The R5 is not just incrementally better, it is a major leap. BTW, there is a rumor that there will soon be a major firmware update that will give the R5 all the latest tracking technology available on the R6 II and others.

I have sold to KEH, MPB, B&H and on ebay. I start by checking what items actually sold for on ebay. After commissions and fees, you will get about 85% of the selling price. Then I check all the major companies. There are often wide disparities. One or more will often offer a surprisingly good price. I sold my 5D4 to B&H for what I thought was very fair. Some will give you more if you are trading in. This year, ebay will be sending 1099-K forms to anyone selling more than 600. a year.

We are in the central mountains--Buena Vista.

I kept my 5D4 for over a year but it collected dust, so I sold it. I have the RP as a backup. Someone else mentioned you might want to sell it and buy a cheaper R mount with the funds. I think this is good advice. I would be all over the new R8 if I didn't already have the RP. I used to own the 70-200 4L IS and found the AF to be its weak point which is why I ended up with the vII. The RF version will be a big upgrade for you.

BTW, I used to own the 300 4L IS and took some nice images with it but I found the AF somewhat frustrating so I bought a used 300 2.8L IS II which I have to say was a revelation on how good a lens can be. On the R5, the AF is so fast and so accurate that I sometimes wondered if it was working at all!

I bought a RP and R5 about 18 months ago. The RP is used for long hikes and climbs. I had 12 or so EF lenses at the time. Like you, my first RF lens was the 24-105 4L which I use almost exclusively on the RP. My EF 24-70 2.8L II has been and still is my workhorse. I would like to have the RF version but he only benefit is IS and it would cost me at least 1500. to make the swap, so I'll wait. All the EF lenses work better on the R5 but some more than others. So far, I've only "upgraded" two. Nine months ago, I replaced my much loved and much used 100-400L IS II with the RF 100-500 which exceeded my expectations. The 100-400 didn't benefit that much from the R5 compared to other EF lenses. With the 100-500, I got lighter weight, insanely good eye detect on birds, great tracking, and much faster, consistent accurate AF vs. the 100-400. See if you can try one out vs. the Sigma. I think you'll be blown away. I think you would love it for your hummingbirds. About five months ago, I replaced my 70-200 4L IS II with the RF version. You mentioned hiking (I'm in CO) and that was my primary reason for switching. The EF version is a great zoom but eye detect didn't work great and it didn't track great either. As you know, the RF version is much shorter and lighter. The reduction in MFD also helps for taking wildflower shots. It works great on the RP too, so it is now my preferred hiking lens. You mentioned the 135 2L which is one of my favorites and it did greatly benefit from use on the R5, especially the AF. Having said that, I did preorder the RF 1.8 IS version. There are too many improvements to ignore. I rarely use wide angles and sold my EF 16-35 4L IS. I did buy the RF 16 2.8 which I will carry in my pack for instances when I feel like playing with the wide angle. This isn't practical with a heavy zoom. I have the EF 85 1.8 and don't plan to replace. I've read good things about the RF 85 2.0. Nothing else is on my radar now. There are rumors of some pretty innovative lenses in the pipeline, so I would rather buy something I don't already have vs. a replacement.