

fail2ban is mandatory equipment for any ssh server accessible to the public especially on its default port. It’s highly configurable, but the default settings will do fine at making it statistically impossible for any user or password to be brute forced.
Nextcloud Notes or Joplin (nevermind all the other features Nextcloud provides) tick most of your boxes. They’re more productivity focused than privacy focused, it doesn’t do “zero knowledge” encryption the way you’re describing, but I don’t really understand the point of that when you’re self-hosting and the server host belongs to you anyway. The federation may leave you wanting more and the collaboration might not be “real time” enough for you either, though. If you can build something better by all means go for it.