Mattol

Are you aware of this project: https://github.com/NixOS/mobile-nixos ?

Is Django not FOSS?

Interesting discussion in the article. Seems like it's a pretty experimental therapy atm without any conclusive evidence that it works.

And Rocketchat and Gitter (now also matrix)...

It is. I tried using it for a larger project as replacement for Google docs and mostly that's fine but we needed sheets and later also appscript to communicate with email so that we moved back. I'd highly recommend cryptpad though for anything that has slightly less advanced requirements.

Cryptpad is using a custom, zero-knowledge Version of only office. They describe the relationship between both here: https://docs.cryptpad.org/en/FAQ.html#what-is-the-relationship-between-cryptpad-and-onlyoffice