I can second using Vimwiki! I have mine set up to automatically sync with Gitlab every so often too, so it’s automatically backed up.
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/