This is the first time I’ve heard of mTLS. Sounds interesting, any tutorial recs?

Just myself, but I would like to keep it synced between my phone and my laptop while also keeping a backup.

I use a similar set up. Librewolf on Linux and IronFox on Android. You could still use a Firefox account to sync, but I wouldn’t. I’ve heard there’s a way to host an older version of Firefox sync locally, but I haven’t looked into it.

Kind of similar, I use Joplin which uses markdown syntax. Then I back that up on a monthly basis to my server manually (just a copy-paste over NFS)