CryptPad is absolutely fantastic. Easy to host and secure design.

Stalwart is 95% awesome. What holds me back is, that Mails are stored in a Database and not Maildir. Maildir is insanely trivial to backup incrementally and to restore individual mails if necessary. That currently holds me on dovecot.

It’s more comparable to Snikket. Both Snikket and Prose use Prosody as server with their own extensions.

You could look into prose. The interface of slack/discord/mattermost, built on XMPP, with E2EE.