We're using the Nextcloud Collectives app and it's great. It's like a very simplified version of Notion but where every page is a markdown file. There's also a full permissions/user system inherited from Nextcloud itself.

The only feature I miss from Notion is the ability to publically share a page so that people outside your team can read it. But that's not a big deal.

I guess Obsidian works in a similar way (markdown files) and would have more features, but there's no user permissions system. And it's closed source.