A place to share alternatives to popular online services that can be self-hosted without giving up privacy or locking you into a service you don't control.
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Gonna take this in a different direction. Sounds like you mostly want notes with attachments and organization, but you're just using chat apps for it right now.
Joplin does all the things you're talking about, including multiple backends and encryption. No need to host a server.
I use SimpleX for this matter. Each device get its own account, and I just create a group named "self" with all of them. You can selfhost the server too if you prefer that route.
Have you consider using a note taking app with server sync instead of abusing a chat service for the same thing?
Since i use the self-chat on Telegram a lot i've wished for exactly the same thing in the past as i'm planning on leaving the service, so far haven't found a right fit yet
How about pushbullet? It's not self-hosted though, but syncs nicely between different phones and browser plugin. File transfer etc. works well, the search is rudimentary.
Honestly I'm not sure about offline support though, I haven't really been in a situation where I wanted to take a note/message myself without internet connection.