Joplin or Bookstack are probably the closest. Joplin is closer to Evernote.

https://joplinapp.org/

https://www.bookstackapp.com/

Yeah I don't think Joplin has collaboration features. I also found the UI to be extremely bad on iOS.

BookStack doesn't have any client apps — it's basically a PHP app like WordPress that you can self-host, but the collaboration feature set is pretty good. It's a wiki that feels a bit like Notion.

Outline could be an alternative if your main usecase was internal team documentation. I understand that Notion can be used for a lot of other things too…

It is BSL licensed, the only restriction is that you cannot run a hosted version for other organizations to use (aka compete with the only way the project maintains itself).

https://www.getoutline.com/

I cannot use outline unless I have Slack or a Google account?!

That's correct.

I'm a happy user of QOwnNotes and Markor with a shared dir with a sync tool of your choice. Indeed, committing my data to a closed (source) silo is what prevents me from using tools like this (Notion does look very neat).

I use Tiddlywiki. The good part is that, once you familiarize yourself with the internals, you can customize it any way you like.

https://tiddlywiki.com/

No. There are tools doing similar stuff of some aspects of Notion. But the main benefit of Notion is the interface and collection of ability. And there is nothing like that at the moment. But the question is what you really want. Maybe you are just care for some specific aspect, not the whole tool?

emacs + orgmode

It's a wiki. MediaWiki has a lot of dynamic features now.