I'm one of those thinking about jumping ship....The lack of resolution on the R3 make that a very hard sell to me.
I can certainly understand why you think the 24MP of the R3 is too limiting, although I doubt if you’d notice the difference compared to the 30MP of the 5DMkiv or the 20MP of the 7DMkii.
- The lack of in-body GPS (the only one that works) on the R5 make that a no-go for me as reliable GPS is essential for my work.
It only takes a moment to get a GPS reading on a phone, so why is it so important to you, to have in-body GPS? If you are concerned about matching the camera images to the phone coordinates, why not just take a photo of the phone's GPS readings as soon as you arrive at a new location?
A zoom that works in the other direction, swapped focus and zoom rings on lenses, button positions etc. seems minor in comparison to relearn to be fair.
Some people can adapt to new cameras more quickly than others. Personally I hate changing cameras more often than is necessary, due to muscle-memory issues, and of course the cost.
If you shoot landscapes or other subjects where you can take a bit of time, switching to a different control system probably wouldn’t be a problem. But if you shoot sports, wildlife or other active subjects, you could easily find yourself missing once-in-a-lifetime moments when you turn that zoom ring the wrong way, or fumble to make some other necessary adjustment.
AFAIK there are EF to Z mount adapters just as there are adapters for EF to RF so the very expensive glass should still be usable to some degree. But even with a switch the RF I'd be looking at (eventually) replacing glass over the next years anyway - I don't like adapters at all. The more stuff you have in there, the more that can go wrong.
Adaptors vary hugely, so it’s important to choose a body/lens/adaptor combination that works well enough for whatever subject matter you shoot. EF lenses works perfectly on R bodies, although RF glass definitely offers improvements in AF speed and better stabilisation.
The most recent Metabones EF-Sony adaptors seem to work very well, but I’d say that using EF lenses on a Nikon or even a Panasonic was pushing your luck. Much of course depends on whether you are shooting action or passive subjects.