I don't see a lot of chats about running vanilla OpenWRT on the Brume 2 (I saw a report back in 2023). In case that's useful to anyone, it's listed as officially supported so I gave it a try.
Functionally, it seemed to work. However, my throughput went down from 600Mbps to 20-40Mbps. I also gave ImmortalWRT a try to see if it would work better, but the performance was similar. So, I'm back to the GL.iNet firmware.
TLDR: vanilla OpenWRT boots up and works, but exhibited terrible performance.