I have the R6 Mark 2 with the 200-800 and enjoy it. That said, I do wish there were move pixels there for bird shots, but the autofocus is fast and tracking is good. The R6 will do better in lower light than the R5 due to the smaller sensor, but you give up pixels.
I personally am waiting for the R7 Mark 2 in early 2026 with an eye to use that for shooting birds, and think that's your logical next step. I'm hoping it will be in the same price range you're looking at or a bit less and that they will improve the controls on the body and upgrade the sensor. That's all I need - stacked sensors would be a pleasant surprise, of course 🙂
If I had to buy just one today, obviously I'd choose the R6 Mark ii for birds. More of my shots are wildlife, though, not just birds. I would probably pick one up used with an eye towards selling it for the new R7 Mark ii, though, as I think the 200-800 on an APS-C sensor like you have with the R10 is a great fit, and the R7 will be even better. Financially, though, just waiting for the R7 Mark ii is probably your best choice - hopefully it will come in at about the same price as the current R7. And who knows, really WHEN it will be released.
If you're going to keep the R10, then the R5 is a great complementary camera. Buying used other than refurbed from Canon has those risks that you have to factor in. Sorry, maybe this isn't much help in making that tough decision!
