I hope it is going to be ok. Automattic has been involved with matrix before. Time will tell, but I’m more hopeful than afraid.
You’re sure you’re talking about OnlyOffice, and not OpenOffice. OpenOffice and LibreOffice are related. OnlyOffice is not.
I tried to compare some backup solutions a while back: https://hedgedoc.ptman.name/kket4uo9RLiJRnOhkCzvWw#
Forget about docker. Run caddy or some similar webserver that is a single file next to the assets to serve.
How full is your ZFS? ZFS doesn’t handle disk filling and fragmentation well.
Quite far: