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You could always check out AnyType: https://anytype.io/

Open-Source, E2E encrypted, offline first, syncs to all your devices via IPFS so no centralized server, etc.

No usernames or email addresses either.

Is that really open source?

I can't find it's source code on GitHub.

Ah, good point, sorry, it's just released in Public Beta this month, and opening the source code is Coming Soon™

Seems that they will take the proprietary source available approach instead.

I don't think that is the same thing as "open source".

Turns out they picked a source-available license.