I am with you on the S5II, fantastic in every way, but too late for me. Once you buy that first RF lens you are locked in way more vs an adaptable EF lens. Not sure if it matters to you, but one big thing the R5 is lacking vs the R6II and R7 is the hybrid hotshoe for audio.
I think I got my quirks worked out with that final EVF setting that was added as an option in one of the FW updates. Also, it is winter here in FL so I have been using the R5 more for video. I just completed a commercial shoot and used the R5 as a B cam to the C70 instead of the R7, mainly because the R5 already had the 70-200mm on it and I needed that lens for the shoot.
The R5's IBIS is definitely amazing, I can easily handhold the R5 with the RF 70-200mm at 200mm and it looks like its on a tripod; not for long periods but enough to get the 30s or so that I typically need. In fact, the only camera that I've ever shot with that has even better IBIS ironically is the R7 (not counting GoPros). The R7 is the first camera I've ever shot with that I can walk for brief periods and it looks like I am using a gimbal when paired with the Canon EF 24-105mm F4 IS L lens. For the commercial shoot the only lens I used on the R5 was the RF 70-200mm and it was all handheld, no problems getting the tripod like stability I needed for all of the shots.
As far as vertical shooting goes, I really dislike small cameras and I shoot vertical (for photography) about 90% of the time since I am mostly filming people, so the battery grip for the R5 really fixes everything for me (makes it bigger, adds battery life, adds vertical shooting buttons).
IMO, I would definitely pick the R5 over the R3 any day, and just add a battery grip to it (which is exactly what I did). Another feature that I really like for photography with the R5 is that those 45MP files when using cRAW are actually smaller than the 32MP files that used to be produced by my 5DIV.
To me the R3 is just too focused on sports/action and you pay a lot for that. It does have some great video side benefits and of course I've never heard anything about overheating with the R3. I'm still not 100% convinced it is fixed in the R5 for Florida summer heat.
Its kind of funny, I have kept the R5 in 4K HQ mode since I got it (which disables and hides all of the 8K options in the menu), so I never bothered looking at the 8K options, I thought it only shot 8K RAW, but somewhere mixed in with all of those FW updates it now has RAW, RAW Light, ALL-I, IPB, and compressed IPB for 8K. Unless you have an M1 Mac, you would definitely need proxies for the compressed 8K files. 8K RAW though I can edit with no problems in DR.
Below were the available recording times with a 51GB CFExpress card in the R5 when set to 8K-D (8192x4320) @ 29.97FPS or 8K-U (7680x4320) @29.97FPS:
8K RAW - 24m 19s
8K RAW Light - 37m 15s
8K ALL-I - 49m 55s
8K IPB - 1h 35m 32s
8K Compressed IPB - 3h 11m 11s
The 8K compressed IPB almost looks small enough to be useable to me, I might have to try it out one day on a shoot.