I have found limiting the use of extenders to the 300-500 range does limit its usefulness quite a bit. It's also more awkward to carry. Given Canon's excuse for not making the 70-200s and 135 compatible with extenders, I assume weight is the reason here as well. I'm OK with that. The size and weight of the 100-500 is a major factor making it a joy to use.

If you are concerned about the small aperture, don't be, there are noise reduction tools from several companies, including PS, that work great. I need the small aperture for DOF reasons if I'm close to a small bird.

I've used the 100-500 for a couple of years. If I were making the choice now, I'd still go with the 100-500. It's better made, has higher quality elements, a little better IQ and is a lot lighter. I've watched a number of reviews of the 200-800 (including head-to-head comapros) and it looks pretty impressive for the bucks for those who really want the extra reach. Bird shooting season is at hand I'll be assessing how much I might use a 200-800. It's possible I would buy one as an addition if I think I'll use it enough to justify adding it. I think I'm less than 50-50.

The AF in all my EF lenses worked better on the R5 to varying degrees. However, the RF lenses work better still so I would conclude there are improvements in cameras and lenses. BTW, my EF 135 2L gained the most. That lens was transformed. The 100-400 L II showed minimal improvement which was a disappointment and one the reasons I went for the 100-500.

You already have a great camera. Why not invest in better glass? The R7 has mixed reviews regarding AF performance. It probably won't work well with your Sigma. If you watched Wegener's videos, you already know the 100-500 is a better lens overall. However, I was very impressed with the performance of the 200-800, especially for the money. I shoot a lot of small birds and rarely need more than 500mm on my R5. Long shots result in atmospheric distortion and haze, so I find them unappealing anyway. I have a 1.4X but seldom use it. The R5/100-500 combo weighs 5 lbs and I can carry it for miles comfortably. The R5/200-800 is probably around 7 lbs. I also use the 100-200 range quite a bit. The eye detect and tracking is like cheating. Almost every shot is precisely focused. It also has a short minimum focus distance and good magnification which I find very useful. The choice, it seems to me, comes down to whether you really want the extra 500-800mm with its attendant compromises. Either one is a great choice. I plan to pay attention this summer to assess how often I could use more reach and decide whether or not to add the 200-800.

People worry too much about high ISOs. There are a number of noise elimination tools available, including PS, that do a great job. I saw a video with Wegener using 12,800, 25,600 and higher. After using the tools, you'd never guess the ISO was so high.

I am cured with an eye for detail and use a R5 + 100-500. The AF, tracking and eye detect are insanely good. I suggest you check pro reviews on line. Throw in the Tamron G2 while you're at it. See which one fits your needs/budget.

I have an EF 1.4X III and RF 1.4X. I used the EF version on a 100-400L II and 300 2.8L II IS. It worked pretty well on the zoom and better on the 300. The RF version is used on a RF 100-500L. I also had an EF 2X III for a while but I found the results unsatisfactory. With any of them, you get some loss of AF performance and IQ, including some loss of color brightness. Counterintuitively, they work better when the subject is fairly close like on small birds. Any long shot will suffer from atmospheric haze and distortion anyway. I've seen pros generally don't like to use them except when really necessary. Jared Polin did a comparison with the 100-500 where he compared the 1.4X vs. cropping and proclaimed cropping is better. Some might disagree. I think the 1.4 is somewhat useful mostly on small birds. Anyone expecting miracles on long shots will be disappointed.

I am cursed with an eye for detail but have to admit today's top zooms are REALLY sharp. I have a bunch of primes but I've been using zooms more than I used to.

But resolution is not the only important aspect of a lens. Sometimes, a wider range of apertures is more important than a wider range of focal lengths. IMO, depth of field is the most important variable in how your images will look. When I'm in the mood for experimenting and creating special images, I usually reach for my RF 135 1.8L IS. I know this is thought of as a "portrait" lens but it is much more useful than that. Even when stopped down to, say, 5.6, the images still have a magical quality that my 70-200 can't match. Subjectively, I have found that primes seem to produce more compelling images, even when they aren't any sharper than some zooms. Some, like the EF 135 2.0L, gain almost cult like status.

With the R mount, Canon is clearly working harder on zooms and delivered several innovative lenses with more rumored to be coming. Some people believe they've been neglecting the prime lens lineup. Prime lenses may be going the way of stick shifts.

I was answering the OP, not you!

I replaced my EF 100-400L II with the 100-500 20 months ago. I agonized over the switch for some time before making the plunge. My only regret was not doing it sooner. The 100-500 is better in every way, especially AF precision and tracking. Plus you get another 100mm and lose a 1 lb. of weight. I use a R5.

Check out reviews online of this lens on various camera bodies. There are reports the R7 AF lacks some consistency.

No rush. Make the transition an enjoyable journey. I took two years to replace 5 of the 12 EF lenses I had. Upon reflection, the RF lenses are a joy to use and generally better than I expected. A few of the swaps were painfully expensive but I see top quality lenses as very long term investments.

I "transitioned" from a 5D4 and 77D to a R5 and RP 28 months ago. I had 12 EF lenes at the time. Over the next two years, I slowly bought 7 RF lenses replacing the ones I use most. I'm now standing pat but will likely buy the new 200-800 at some point. I sold everything I replaced except the 135 2L because it takes an extender which the RF version does not. It's also not worth much.

It sounds like your daughter has no spare time now, so I would sell all the DSLRs. They are just sinking in value while you wait. Ditto the EF lenses you've replaced. Use the money to buy a R5. You'll be glad you did. You can buy her a shiny new mirrorless when she graduates.

yes

What exactly do you want to do with it that you can't now?