Proton is making me more uneasy with each of these acquisitions. I don't like companies that get to big 😅.
I'm using Joplin notes with a self-hosted Joplin sync server. Excellent customizability through plug ins and CSS templates that you can find on the Joplin forum.
I looked at standard notes back in the day. The one benefit it had over Joplin was that the notes could be encrypted on the client devices, while with Joplin you can only encrypt the notes on the sync server target. Other than that though, Joplin checked many more boxes, personally I can't recommend it enough. I use markdown for formatting, I can't speak to the quality of the WYSWYG editors functionality.