This seems like something that would really benefit from better language support. I saw the translations folder in the repo, but you should probably get it linked up to a Weblate instance or similar and have people start contribute different languages asap.

Collabora Office works independent of Nextcloud. It can be nicely integrated into the lightweight https://www.filestash.app/ frontend for example.

I recently ended up replacing OnlyOffice with Collabora Office (Libreoffice based). While architecturally less efficient that OnlyOffice in theory, for my usecase where I rarely check some documents in Filestash it actually ended up using less server resources when idle than OnlyOffice (to my surprise).

UI wise they is no big difference IMHO, but docx etc. compatibility is slightly worse.

XMPP obviously. Have a look at https://joinjabber.org/

This is a fundamental issue of the Matrix protocol, yes. For regular small scale use it doesn’t matter so much, and the state history gets reset every time you do a room upgrade, which is another annoying “feature” of Matrix, but it eases the fundamental problem a bit.

But IMHO the Matrix protocol is a child of the Bitcoin hype era and is built on a similar data-structure that is inherently impossible to scale and the developers of Matrix should have realized that early on. Their bosses back then actually did, but they spun it off as a separate company and got some crypto-currency investments so the can was kicked down the road and here we are…