The simple answer is: RF. Mirrorless is the future, and this is true across the industry. Sony have E-mount, replacing the old Minolta/A mount; Nikon are dumping F for their mirrorless Z mount; the Four Thirds system is a distant memory, having been replaced by Micro Four Thirds; etc.
The problem is that RF is relatively new: there are quite a few RF lenses available, but they tend to be expensive. Third-party lenses are manual focus only; there are no Sigma lenses for RF. Give it a few years and I think it's pretty clear that all this will change; Canon have made it clear that RF is the future, so anyone (Sigma e.g.) who want to play in the Canon market, needs to get with RF.
In terms of cameras, I think definitely RF. An RF camera can take both RF and EF lenses, with an appropriate adapter, so it's the way to go. It would also be adaptable to PL mount, Nikon F, etc.
As for which camera, you say you "like the capability of high quality stills if I need to". Honestly, I think you're spoilt for choice there; I don't know of any decent modern mirrorless camera that doesn't take really good stills. I you want really high-end, then the R3, of course, but really, I think most RF cameras will fit most peoples's stills needs. If there's a specific feature you want, like maybe intervalometer, then obviously check whether the camera you're looking at has it; but otherwise, you can't go far wrong.
But you also say "I shoot mostly video". I don't know if you're looking at fairly casual video -- in which case, again, spoilt for choice -- or if you want to go very HQ. But if the latter, then I think the R5 and R5C are both worth a look. The R5 (I have one) is a fantastic video camera; it shoots 10-bit, log, and has true 4k pixel resolution. ("4K" cameras have 4k photosites, not pixels, and use interpolation to get to 4k resolution. The R5 has 8k photosites, so delivers a genuine 4k pixels. The difference is clearly visible.)
I've been using the R5 for 9 months, and I've been shooting half-hour landscape segments with it, in 4k HQ mode, with no problems. However, half an hour is the limit for any clip on the R5, and if you keep hitting record when it stops, you may run into overheating issues. The R5C fixes both these problems, so it's certainly an option, though it is bigger, due to active cooling. I also have a C70, which is a whole other league of cinema camera, but it's superb, though its stills capability is basic.
The main thing I would say is figure out what you want, and price out the ENTIRE SYSTEM -- camera, batteries, adapters, lenses, memory cards. Then choose what best fits your needs.
Do bear in mind that while EF lenses can be adapted to RF cameras, RF lenses can NOT be adapted to EF cameras. (You might be able to use a diopter adapter, but all the reviews I've seen say those are junk, so don't go there.)
