Sadly I don’t have a solution for mobile. The truth is that I mostly use it on the computer though so it really hasn’t been a problem.
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/
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.