You cannot stick a compression step before restic, because it will mess up the deduplication - the chunker will not find the same chunks, and will not be able to find out which files have same contents (not as easily at least).

And you cannot stick it after restic, compressing the backup archives (or storing them on a compressing filesystem), because those are encrypted, and any encrypted file has high entropy and does not compress. (It doesn't help that restic makes the encrypotion mandatory).

So no, compression really needs to be an integral step in a backup system. Restic is definitely lacking in this regard.