The proxmox-backup-client will also just copy files/folders and not the ZFS filesystem under it. If you just want to copy your datasets there is always the option to use zfs replication like the pve-zsync you already mentioned. PBS also supports to use a ZFS pool as a datastore, so a PBS with ZFS could be a target for both PBS backups and ZFS replication.
But I don't see a problem that PBS will just backups files/folder. It won't hurt that snapshots aren't backuped because you can basically create as much backups as you like without consuming more space than the snapshots. I don't use ZFS snapshots anymore since I got the PBS running, because snapshots just waste additional space. I just moved from weeky Vzdump Backups + daily snapshots to daily PBS backups without any snapshots and now I'm saving alot of capacity with that. PBS also got checksumming, also got compression, also got encryption, also got deduplication (and that without having hundreds of GBs of RAM) and it can sync between differnt PBS as an alternative to replication.
