I'm late to the thread but borg or restic or any of the attic/bup/Xbackup lineage will meet your needs. I haven't tried the GUIs but you can schedule it to run

I'm late to the thread but borg or restic or any of the attic/bup/Xbackup lineage will meet your needs. I haven't tried the GUIs but you can schedule it to run with anacron once per day and it'll run when the laptop first turns on. You can sleep 10 minutes before running to give yourself a chance to get connected. Because of how the repositories work, if a backup fails due to list connection the next one will use that partial data so the second backup will have less to transfer. NetworkManager knows the metered state of the wifi and so that can be read in a shell script to decide whether or not to run the backup. You may want to whitelist known good wifi networks instead. Perhaps someone could take the excellent Deja Dup GUI, which is a front-end for the technically inferior duplicity, and have it shell out to borg instead.