There are two angles to this question. The answer depends on whether you plan to print or not. If you plan to print I would crop it to the size you plant to print at, like 8x10.

The second point is that this is a matter of personal preference, particularly for viewing at the computer screen. I personally prefer the 3:2 ratio, but I have cropped images at different ratios for various reasons--sometimes even a 1:1 ratio.

I have photographed dozens of church interiors in numerous countries (including Italy), and can say with confidence that 18mm full-frame is wide enough, but 18mm APS-C is not--because de facto has the field of view of a 27mm lens. (This is why I recommend taking the 10-20mm APS-C lens.)

Many folks here throw around numbers of focal lengths without specifying what format they are talking about. This leads to a meaningless discussion.

An example here is the OP's option to take a 35mm lens--52.5mm full-frame equivalent--which many here endorsed. While having a faster prime certainly can be useful for night shots, 52.5mm is too long to be really useful for this type of photography in my experience. 35mm in a full-frame system would be a different story--that would be in the 20-24mm range for an APS-C lens.

I would take the 18-70mm and the 10-20mm lenses (full-frame equivalents 27-105mm and 15-30mm). Nothing in your trip indicates that you need a long telephoto lens. You will mostly use the 18-70mm lens. However, a 18mm (27mm equiv.) focal length is not really wide enough for many architecture and landscape shots, so I highly recommend taking the wider 10-20mm lens.

In my Nikon D7100 days, I used to travel with 18-140mm and 10-20mm lenses (including a trip to Italy). This worked great.

These days I travel with a Sony full-frame 24-105mm lens and a full-frame Zeiss 18mm prime lens. That is what I had for five weeks of travel in Spain, France, Switzerland, and Germany this summer--that is all I needed.

This is the critical point. The true focal length of the ZS50 lens is 4.3mm-129mm. The true focal length of the Sony 18-135mm lens is 18-135mm. Focal length refers to the optical properties of a lens. However, the sensor size varies--the Lumix having a tiny sensor, while the Sony has a much larger APS-C sensor. Sensor size matters because bigger sensors give you a wider picture (field of view) than smaller sensors--this is why we are talking about crop factors.

For this reason, it is impossible to directly compare lenses. This is why we use full-frame focal lengths as point of reference--hence the word "equivalent." The ZS50 with a large crop factor gives you the field of view of a 24-720mm lens, while the 18-135mm lens with a smaller 1.5 crop factor gives you a field of view of a 27-202.5 mm lens. For this reason, the ZS50 gives you a slightly bigger field of view than the Sony 18-135mm lens.

As others pointed out, with a good adapter you have full functionality for modern autofocus lenses on a mirrorless camera, including autofocus and aperture.

You also mentioned older manual lenses. I have used manual lenses from the 70s on my Sony mirrorless bodies. While there are no electronic contacts, the camera reads the light that comes through the lens, given the aperture setting you chose manually. It automatically adjusts time of exposure and/or ISO, depending on your setting. It is like shooting in aperture priority.

The camera automatically goes into manual focus mode, so you use focus peaking to focus. It is really easy to use vintage manual lenses. Adapters for manual lenses are cheap as there are no electronic contacts involved.

I have been using the VSGO sensor cleaning kit on my two Sony mirrorless bodies. Tony Northrup has a great little video where he shows the procedure. It is very simple. I have done this a number of times myself, without problems.
https://www.youtube.com/watch?v=aAiTy3qGg2Y

I use my rocket blower on my sensor after each use, before I put the camera away. This greatly minimizes the need for a wet cleaning.

Click bait? The title says it all: "Moment's new smartphone camera lenses rival Sony's GM pro glass." In good light, smart phones take good pictures, as long as you don't blow them up too much. I travel with two great Sony camera bodies, and I still take pictures with my smart phone. What is unfortunate about that?

The answer will depend on what you plan to use this new lens for and what lenses you already have.

The Sony 200-600mm is a great wildlife and birding lens. However, it is large and heavy. It is not a great all-purpose telephoto lens--it is too long for that, and definitely will not be useful as a portrait or real estate lens.

I use the Sony 100-400 GM lens as a wildlife, nature, landscape, and portrait lens, and I love this lens. It is considerably smaller and more versatile than the 200-600mm lens, and with a minimum focus distance of about three feet it has good macro capabilities. While a great lens, it also is a bit more expensive than the 200-600mm.

The brand new Sony 70-200mm f/4 Mark II might be a good fit for you. It certainly would be useful as a portrait lens and as a zoom for general photography. It also can be used as a macro lens.

If you have the budget, the simplification alone will be worth it. The Sony A7RIII still is a great camera though--you might keep that as second body. When shooting wildlife, changing lenses may not be convenient. Keep the long telephoto lens on the A7RV and the standard zoom on the A7RIII. The A7RV will give you ample megapixels to crop for wildlife so you will not miss your Nikon D500.

Flickr had the hiccups lately. The same thing happened to. I just waited a couple of hours, and everything worked fine.

Seriously? How can a photographic device be "disgusting"? The only thing that is "disgusting" is your reply. How about trying to come up with civilized posts that include at least an attempt at analytic discussion?

I travel with a Sony A7III as well, and I am no spring chicken either. So size and weight of my equipment are important to me as well.

The Fuji X-T5 weighs 557g, vs. the A7III at 650g. The difference is about 93g, or just under four ounces. There is a difference, but it is not huge.

The difference comes in with lenses. My main travel lens is the Sony 24-105mm that weights 663g. To me, a travel kit of 1,313g is acceptable, but it may be too much for you.

I would consider the new Sony 20-70mm that weighs only 488g; the reviews for this lens are excellent. To me, this is the perfect focal range for travel, and you would not really need to bring another lens. I am proposing this as a possible solution that will allow you to travel light with the camera that you already have.

I do. I am a Sony shooter, and I am happy to see Nikon coming out with a great camera. Competition spurns innovation and keeps prices in check. That is good for all of us.

In the "good old" film days we first had to develop the film and make a print to see if the pic turned out. So DSLRs presented real progress.

Upgrading to mirrorless would solve this problem.You can review your images through the viewfinder, rain or shine.

That's how it goes here. Long after the question at hand is settled, the curmudgeons come out and blabber on for pages and pages. This site is barely relevant for photography questions anymore.