Assuming you want open source for your private notes, then NotesNook and StandardNotes (premium SN is not open) are great options. Personally I prefer SN.

If you already use vim/nvim, then continue using and sync with SyncThing which works perfectly. Only downside is for phone you have to use a different text editor such as Markor.

Obsidian is very slick, high functionality and seamless, BUT it is not open source, so I wouldn't want my private notes in there.