And I heard that if you self host you can use the premium features for free

Agreed, me to! And I use syncthing to sync my database between my devices Edit: mine is called KeePassDX but its the same database file

Peer2peer connections are very difficult. Mainly because clients are behind a NAT which allows only connections from inside to outside. So when a client wants to connect to another client one connection comes from outside and is blocked. There is a “bug” called NAT holepunching to still connect but that’s not a trivial task and doesn’t work on all setups. I recently tried out webRTC for a multiplayer game but I still don’t know what to do with people where that connection can’t be established. SteamAPI has a P2P feature as well, and when they can’t establish a p2p connection they just silently route the packets over their servers.

Just today I learned about Cinny which would make so much more sense to use for people who need the discord/slack experience but want to use matrix

Rock and Stone!