Besides the @cdhowie option, you can also use restic with Termux. I’ve tried it (not extensively) and it works.
Edit: I think I got to work the arm64 version if I’m not mistaken but you have to make sure to give Termux permissions to storage. If you have your phone rooted, I think it could be possible to backup the root directory; if not, you can access to /storage/emulated/0 and backup everything there which could be more than enough since that’s the directory that contains basically everything. Also, you can use cd .. in Termux and right there you should find home and usr directories. Navigate to /usr/bin and put the restic binary there for convenience.