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.

Interesting article! Always a pleasure to read other people journey, from an idea to something tangible.

I think going the "WordPress route" could be the successful way for a project like this.

(But I think, for now, adding "Fediverse features" could potentially create caos more than benefit)

Agreed with above, Wordpress ecosystem type sounds promising