I run Nextcloud, but back it up with protonmail. I auto backup my Obsidian notes to Nextcloud to integrate it, but I prefer HedgeDocs for he Google Docs-esque documents.

https://docstr.app/ is pretty good, but I haven't tested it enough to really vouch for it. I haven't even tried private documents.

Keet is the easiest way to share files between all your own computers, but I'm not sure it's conducive to collaboration just yet.