I think I am now in a position to comment on this with some authority after the purchase of Canon 5Ds, which is at 50MP or slightly more than the R5. On tripod there is almost nothing between them. Of course Sony, Nikon and Panasonic have their own high MP models.
Before that I had (and still have) 5D mark III. They are very sharp with all decent and proper lenses, like anything from that last decade with red ring on it. Specifically 16-35 f/4, 24-70 II, 100mm macro, 70-200 f4 IS, 400 5.6 + Sigma 35. Pretty much the problem areas were only 16-35 at 35mm in the periphery and that is ever so minor.
On 5Ds things change substantially. Centre remains pin sharp with careful focusing with pretty much every lens above. I must stress critical focus again. I had a few slightly misfocused 24-70mm shots particularly in very low light and they look rubbish until downsized to 22MP, where they look perfect. The far edges on 24-70mm pretty much leave me wanting more, a lot more... 16-36 is still great at 16 but at 35 I would prefer to swap lenses, ideally for the Sigma. Sigma is the only wide lens in my kit that is pixel perfect down to corners at f/8 (and perhaps 5.6).
Long lenses show a whole different story. 70-200mm looks nearly perfect except for more pronounced CA. LR deals with it seamlessly though. I haven't tried 100 yet, but don't expect anything unusual. 400mm is a corner to corner perfection if you avoid any shake.
So no 1. it seems these cameras love primes and long lenses. There is nothing wrong with cheaper zoom like 24-105 but you are basically reverting back to 2XMP images outside of centre part, while clogging up your cards, hard drive and so on.
No 2. is all about critical focus and depth of field. This applies for landscapes, macros, products anything where entire image needs to be sharp. I noticed it is next to impossible to get sharp landscape with foreground at 35mm, and to a lesser even 24mm presents problems. Thus the solutions is a focus stacking nightmare or tilt and shift lenses where one is even available. Good luck with 35mm or 70mm on Canon!
Handheld expect to use at least double shutter speeds. Even with IS and IBIS you still face the exaggerated subject movement.
The cameras enable very high res shots to be recorded, but at the same time the process becomes no longer fun day out like with 2XMP camera but quite a PITA if you try to make the most out of it. Worst case it won't be any worse than the lower MP alternative, but it might not be much better either without extreme care.
I don't have one now, but from experience this would be terrible until f/2.8 but absolutely stellar at f/5.6. I think the cheaper and better made 1.8 STM version makes more sense with those considerations.
I'd say the Sigma Art series would be good enough
If you don't mind stopping down a bit and swap them every 5min it will be hard to beat. The 35mm wide open to f/2.8 does leave a bit to be desired in a landscape setting; this could be focusing accuracy in the bloody darkness too.
For general purpose or all-rounder I think something between 20-30MP makes most sense with FF sensor. That easily gives you clean A1 prints already. 50MP can fill the whole wall, or I suspect is a convenient number for 8K video capture.