Stuff like this should of course be open source, with the optional possibility of self-hosting. E.g. Nextcloud style.
But, the bigger question is, for something shared here on hacker news where an abundance of developers dwell: why would we not create our own ~500 lines note taking synchronization function with built-in end-to-end encryption?
I mean use your favorite scripting language, even on your laptop, and you could theoretically make something that is very easy to audit and maintain. There is no need for extreme complexity with thousands of lines of code (or untrusted closed source software for that matter). Just something simple to keep your notes encrypted.
Oh wait, we got KeepassXC already (although that is a very complex program and hard to review for us outsiders that do not know the code. Etc. Etc.) I personally do not have time to review such code, and so, it remains a matter of trust for me. I still prefer it because it is open source.