One option for you that's half way there is to use Obsidian on top of markdown files and sync those with Syncthing.
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.
I'm building a plugin for Obsidian called Relay that makes Obsidian real-time collaborative [0].
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.
I love this setup. I've now synced between my phone and laptop, with a backup on a cloud server, and it deserved a post on my fledgling blog: https://blog.sahil.ink/obsidian-and-syncthing/
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.