I was looking into this some time ago and wondered why it's so hard to find a solution. It should be super simple thing to self-host a basic server that can manage document editing. Yet there seemed to olny be suspicious middleman services.
Discussion
You can self-host Nextcloud on Sovereign Computing, AKA StartOS from nostr:npub126ntw5mnermmj0znhjhgdk8lh2af72sm8qfzq48umdlnhaj9kuns3le9ll