I happen to own both of these cameras. They both fit their own needs, so I'm not sure how much this will help, but I'll give you my opinions on each.
I've also shoot RAW and have for years. I also do urban and landscape, as well as sports, and a lot of nighttime photography - both in the city, and with night skies. So yes, the shadows are a big thing with me as well. Both of these cameras do have the same sensor, processor and AF system, so I really won't talk about those other than to say these cameras are great at low light and shadow. Best I've had since my 6D, which for low light was actually better than the 5D series (the 5D series was better in many other ways of course, but not low light). The R6 Mk ll and R8 are much better than the 6D was.
But getting down to the main differences that matter to me, and might help you decide...
Size: Personally I have large hands. Not freakishly large... just average large as in I wear a size large winter glove - so kind of normal. The R8 ergonomics are better for someone with smaller hands. I find it awkward to hold after a while, whereas the ergonomics of the R6 Mk ll is comfortable to hold even on all day shooting events. If you have smaller hands, the R8 might have better ergonomics. If your 5D was comfortable in your hands, the R6m2 will be as well.
Battery: The R6 Mk ll uses the larger LP-E6 batteries. The R8 uses the smaller LP-E17 batteries. Of course conditions vary with shooting and temperatures, but in general I get twice as many shots with the R6 Mk ll batteries as I do on the R8. If I'm doing an all day event on a cold day I'll go through 3 or 4 batteries on the R6m2. Same event with the R8 I would need more batteries than I currently have, which is only 4 for that camera as well. But even for events that are not all day, it's nice to get twice as many shots before replacing the battery.
SD cards: The R8 has one slot, the R6m2 has 2 slots. It's rare that a good quality SD card goes bad, but it does happen. On my hired events that I can't reshoot, that backup card matters. If I'm out doing landscape or street photography, one card is fine because I can go and reshoot if necessary. I did have a Pentax DSLR many years ago with one slot. Went on vacation - got home and found the card went bad. Out of maybe 500 images, I was able to recover less than 50. Lesson learned.
IBIS: There are plenty of mixed feelings on image stabilization. You have the old school guys like me that while we like it when it helps, we know it's not necessary at all as long as you use good shooting techniques, such as a tripod for slower speeds, or shoot faster than 1/250 and the IS doesn't matter at that point. But walking around taking photos at 1/100, yes, it does help (but again - not necessary). Also, if IS is a concern, you can still use the R8 (which does not have IBIS) with a lens that has IS. On my R6m2 I have IBIS turned off most of the time.
EF lenses... many are just as good optically as their RF counterparts. Not all, but many are. While I do have a number of RF lenses since moving over to mirrorless in 2019, my most often used lenses are the EF 17-40mm F/4 L, and the EF 70-200mm F/2.8 Mk l L. A bit heavier? Yes. But even as someone who gets paid for a lot of my work, I can't justify trading them in for the same lens in an RF version when the end results will be the same. It would be like throwing money away.
Bottom line... the R8 is nice as far as sensor, processor, AF, and if you want to travel light (and I do use it often). But the R6 Mk ll is my workhorse.
Gary
Between Digital and Film, current number of working cameras is at 27.
Addiction is a horrible thing.
