If you’ve already got a flint, a Brume is probably redundant, though I could give some reasons. It has more memory and storage, which would more easily let you do things like you’re considering (docker), though you can do them with extroot too. They’re cheap enough that you could get two and run a high availability setup, although that’s some work with OpenWRT.

Personally I prefer to have my router and access points separate so I can upgrade either of them independently. I’ve been running the same pfSense install (upgraded hardware and versions, obviously) for the last decade, and it’s been rock solid. I’ve gone from a single OpenWRT access point to a 5 AP setup in that time, all while keeping the same basic routing configuration. You could do something similar with OpenWRT, though it’s a little more complicated with the variety of packages involved.

At the end of the day I standardized on a different platform that works well for me, but OpenWRT is the king of low power devices (I even run it on an embedded armv5 board in production at work!).

If you were actually going to go that route you’d probably be better off getting a cheap x86 board or something like a nanoPi 6S and virtualizing your setup (OpenWRT or one of the BSD alternatives). That way you can easily move setups between hardware setups. Can describe more if interested.