Vimwiki works great for this! Hyperlinks and diary management, all plain text and able to be opened in anything.
I'm curious if there is something you are using on mobile for vimwiki? I've been using termux on Android, which works well enough.
My backup method is pretty simple but it’s been silently chugging along for months now without a single hiccup. It’s just a git commit via cron job: https://thelinell.com/using-vimwiki/