I basically have the same setup, using nextcloud notes. The notes aren't in plaintext, they are in markdown. Joplin can use nextcloud has a sync backend. There are also other note taking apps that can use nextcloud has the sync backend but in the end I wasn't happy with anything. Going to test drive Obsidian and dendron.so to see how they go.
I'm using https://play.google.com/store/apps/details?id=it.niedermann.... on my phone. It's not super fancy, but it works well. I might try out Joplin sometime, but I'm pretty happy with what I've got.
I'm running it on an unRAID server with nightly backups, but you could just as easily run it from a raspberry pi.
Before next cloud, I was using text files in Dropbox.
My employer just started using notion. It seems fine so far, but I don't see myself switching away from next cloud any time soon.