We have the 1st Gen R6 and R3. The R6 mkII falls somewhere between the two in terms of features and capability. It’s closer to the R3 in capability from what I’ve read. AF is supposed to be about as good as the R3 and it’s fantastic. Resolution is about the same as the R3 at 24ish megapixels. The most common complaint I hear is about the resolution, and what you are left with after cropping and then enlarging. I'll either zoom in with the lens or my feet, but there are times I'd love to have pixels to spare.
Third party support for glass... Allegedly coming, maybe, sometime... Canon has been pretty cryptic, but has said that there will be at least some third party lenses. The interpretations I've heard and read is that lenses will probably be approved on a case by case basis. Far from ideal. We bought the R6 with the expectation that third party lenses were probably a year or so away. We still have some Tamron glass and I expect the 35mm f/1.4 will be in the bag for a long time.
The adapter... It works pretty well. It works better with Canon lenses, with L series being the most consistent. One of the deciding factors for us was the ability to transition to RF gradually, which has worked well for us. Our Tamron lenses tend to have more issues, with the 150-600 G2 having the most and pretty much living on the 70D crop censor for wildlife photography. AF wigged out one too many times at a critical moment. The 35mm has been pretty foolproof. With larger lenses I don't love the balance with the adapter pushing the weight further out from the body. That's more noticeable with the R6 than R3. The adapter works, and it has allowed us to gradually transition. That 35mm Tamron will be in my bag, until I dig dip enough to replace it with a f/1.2 from Canon, and then the adapter probably goes away.
Canon RF glass is the second factor that kept me from pulling the trigger on a Sony a7. Sorry
@fractal. It's nice. Really nice. Unfortunately it's priced like it too
(EDIT: Meaning Canon glass, not the Sony a7. I don't want to draw the wrath out of @fractal ). That being said, I've been more willing to invest in L series gear for RF than than EF. We've been selective, and by checking the Canon refurb page regularly have found some great deals (I still credit/blame you for the 100mm macro we now have). The optical quality is just better generally. The weight is usually reduced. In the first year to 18 months we had the R6 I still had moments when I'd consider switching to a Sony a7, usually due to all of those lovely megapixels, LOL. In the end it was the RF glass that existed and that has continued to roll out. I'm anxiously waiting for our 10-20mm f/2.8 to arrive.
As for switching to mirrorless in general? Zero regrets. Years ago I shot a LOT of film using Nikon and Contax/Zeiss gear, then, didn't pick up a camera until my wife picked it up as a hobby. I dabbled some with her 70D. Mirrorless drew me back in. It's lighter, more compact, and at this point the image in the viewfinder is really good. If I'm on the fence about changing my aperture a stop or not I can see the difference immediately and make a choice just about as quickly. In low light, and low shutter speeds I don't have to worry about the mirror, or lock it up. It is different, and I did have to adjust some but the benefits outweigh the negatives for me. Finally... the EF ecosystem just isn't going to get any better. Development has stopped.
Sorry about the length! All of the above is just my opinion, and worth exactly what you've paid for it. If you have any specific questions feel free to ask.
) or not.