If s3 is the goal, there is gof3r which can be piped into i.e. skip storing mybackup.zfs.bz2 locally.

Overall, I didn't see if they've identified the bottleneck. My guess pbzip2 is the slowest, ssh second. For compression bw I'd check zstd. For ssh there are various cipher/compression options. Or perhaps skip altogether and use wireguard.