Prioritize files/folders

Is there a way to prioritize files/folders, e.g., when I do

restic -r repo backup /home/a /home/b

and I want /home/b to be backed up first?

No. If you want /home/b to be backed up first you need to run a backup command with that folder first.
Restic backups can be resumed in case they get interrupted – see FAQ — restic 0.16.3 documentation