To answer the direct question, BTRFS works fine for gaming. Garuda uses BTRFS by default and I’ve been daily driving it for a few years now. My gaming machine hasn’t had an unrecoverable failure that wasn’t my fault (not checking consple output for errors when updating and then rebooting). Games on an ext4 file system work fine - that’s what I do for games I don’t play often. The main NVME is for games that are played regularly and everything else goes to the storage SSDs.
Correct me if I’m wrong, it seems like you want an immutable distro more than BTRFS for what you want to do.
I use Proton, business tier. My only gripe is that addresses can’t be deleted without contacting support, or so I’ve read. I can’t find a delete button on any of my addresses, but can find the button to buy more address slots.
Using custom domains and a catch-all pointing to certain labels is my workaround.
Yeah same here regarding immutable distros. I’ve only dabbled in the reading and it seems to fit your use case. ^^
Keeping my eye on the thread for future reference. Best of luck!