Here is a list of most banking apps and whether they work or not. I’ve been using GrapheneOS for a while and haven’t encountered major problems. Only thing I wish I knew beforehand is that multi-user profiles are more cumbersome than what most people say.
The B650M is a smaller size and therefore has less features, but overall they seem similar. Biggest difference would be the integrated SSD heatsink and better VRM design + cooling of the non-M version. Also the second SSD slot being connected to the CPU instead of the chipset, if you ever want to put in two M.2 drives. One thing to watch out for is that both of these boards use a Realtek LAN chip which sometimes can be problematic with Linux.