Canon R6 Mark II vs used Sony A7R IV

I think you'd be more immediately comfortable with the Canon than the Sony, as the controls and menus would be a lot closer to what you're used to with the 6D.

But being used to something is no reason, on its own, to make a decision like this.

The Sony has the R6 MkII beat as far as resolution is concerned. So the files will be much bigger. Will that matter to you? Probably only in that you'll have more room to crop if needed. But if you fill your frame, then that doesn't matter as much.

I had a Sony. Couldn't stand it. It felt fiddly. Menus were terrible. I really felt as if I was using a camera that was designed for the engineers who built it, not for the photographers who would use it.

That's just to say that I think you should go try both. Go to a camera store and see what both cameras feel like. Are the buttons where you naturally feel they should be? Are the menus clear and intuitive? Can you take photos without concentrating so much on the camera in your hands that you lose sight of what you're photographing?

We could talk specs all day. People here tend to. But there's more to falling in love with a camera than that. Or falling in like, even. So much has to do with having a camera that makes you want to go out and challenge yourself to take better and more interesting photos.

Circling back to your question, both cameras will suit your needs. Go try them out and see which one suits your soul.

Good luck with your choice and let us know how it goes!