Obsidian has many of the rich editing capabilities, especially when you install plugins. For plus points the files are very portable and there is (almost) no "vendor lock in" because it's all markdown textfiles.
It isn't end-to-end encrypted (yet), but you can self host the document collaboration server on a private network (like tailscale).
If you're like me and you need real-time collaboration and privacy but e2ee isn't a strict requirement for your collaborative docs then you might enjoy it.
I also use Obsidian sync for e2ee device sync -- it is a fantastic product.
[0] https://relay.md
But yeah, since it stores everything as flat markdown files, you can sync or archive your Obsidian docs folder with anything.