Moreover, you don’t gain much by doing this “subdirectory solution”. restic always saves pack files once they are full enough and index files if they are full enough or old enough. That means if you interrupt a large backup, you’ll lose the index contents of the files uploaded the last few minutes, but all other pack files are saved in the index and will be used if you rerun the backup (exactly like in your “subdirectory solution”). If you don’t want to lose that last minutes, you can even run rebuild-index (which has become very fast with 0.12.0) and all pack files will be used for subsequent backups. You just have to make sure that your don’t run a prune which would of course remove this so-far-not-needed data (this is the job of prune).
If you want to gain anything in your “resume backup”, you have to make sure that the files already contained in the repository will not be read and hashed again, see e.g. this thread and the discussion within: