Has anyone suggested a Nextcloud installation? You’d have a notes sync as part of the whole calendar setup. I use QownNotes on my Linux computers and the native notes app on my de-googled Android phone. The phone requires the DavX app to setup the sync, but it’s bulletproof after that. The notes are available as .md files and exportable as pdf.
Nextcloud might seem like overkill, but it is light enough to run on a Pi4 and it would take care of most of your server management and update challenges. You can use as much or as little of its functions as you want.
Okay. I only suggested it because it’s probably one of the best maintained and funded Open Source projects in the world and the caldav/carddav setup is almost automatic. It would allow you to use any caldav friendly notes app on phone or desktop, which means it’s Apple, Windoze or open source friendly.
Generally, I avoid obscure ‘solutions’ and stick to mature tech.
Anyway, good luck.