PandaInSpace

There isn’t a 1:1 FOSS replacement for Notion. I’ve been also looking for one and haven’t found any.

As others have said, Appflowy and Affine are the best options that don’t require self hosting. Appflowy now has a web version too. There’s also SiYuan.

If you’re fine with self hosting there might be a few options mainly for database like functions, some provide a free trial/tier :

1. Undb

2. Teable

3. Docmost

If you want a proper journaling app :

Inner Ink

Linked

Rednotebook seems solid

Having markdown means you can use it in a variety of apps but I’m not sure how many have encryption (obsidian and joplin maybe?)

You could also try Trilium Next

I also found Darkwrite. Can export as HTML