There is a Nostr plugin for Obsidian.

I don’t know of a client specifically that would let you organize things like Notion though. I got really annoyed with Notion because I was trapped in their system with how they do their notes specifically. I switched to the .md ecosystem with Logseq, Obsidian, Roam, and a number of other apps and haven’t looked back. I can open all of my notes in any or all of these apps and just copy folders to save or backup things. No complicated exporting or formatting changes, etc. 

That said, integrating that with Nostr is tricky if you don’t want to share literally all of your notes and/or you want to organize your Nostr notes in a file system like way. 

My basic thinking on this would be to just have a note taking app with Nostr integration automatically save and download every note from your Nostr pubkey by default, then you can organize them from there. 

Lots you can do with this idea though. If I wasn’t focused on other things this would be a great one to tackle