rclone [1] (bidirectional sync) on Termux [2], with Termux:Widget [3] to make syncing as simple as a button press. I backup my stuff to Google Drive, although rclone has many other supported remotes.

[1] https://rclone.org/

[2] https://f-droid.org/en/packages/com.termux/

[3] https://f-droid.org/en/packages/com.termux.widget/