My thoughts as someone who used to do a ton of Exchange/365 setups.

Protocol compatibility shouldn't be MVP. Forcing everyone into a browser is fine, millions of people interact with Google that way.

Same thing with End to End. Just ability to self-host is fine. If someone wants to encrypt their data, since you store it all as files, just have them encrypt local file store.

SQLite + Files is great storage method and actually makes your system more clusterable than you think. You develop a gateway that looks up which servers holds the active user, direct all traffic there. Secondary servers just receive updated copies of data from primary. Exchange works this way.