Earlier quoted context omitted.
You sound like you're describing https://standardnotes.org/ ! (Not linked, just a happy paying user)
Is it completely open source + optional paid hosting (I spent a few minutes reading but wasn't sure.) FWIW: Open Source client + server with optional paid hosting is my favourite. And it is not just words: I've been paying a few hosting services, both commercial and open source, for a few years. I like the assurance of open source but I often prefer to pay the people who make it to host it.
Open source is split across multiple repos:
https://github.com/standardfile/ruby-server
https://github.com/standardnotes/web