This is something we wrote at decent length about in the past — let me know if this is helpful for your use case.

https://satchel.com/knowledge-base/

I haven't tested it personally, but I trust those people enough to take a closer look: https://clubhouse.io/write-beta/

Outline is really good, and can be self-hosted.

https://www.getoutline.com/

For a basic company wiki Nuclino is another option. It doesn't have all the features Notion does, but I loved the incredibly fast page loads and search. Notion feels pretty sluggish by comparison.

If it’s for documentation, BookStack is fantastic and user friendly.

I know Notebag and Obsidian - both are more open-source, Obsidian is almost like a layer on top of your Markdown content you can leave in the file storage

We've been using Slite for a while; it's pretty great.

Confluence works but it's a bit of a beast in multiple ways.

Slab is very nice