I think the ZFS changes were due to needing a way to allow qnap systems to expand zfs pools. The raidz expansion features in openzfs probably took too long for qnap to wait.
This kind of migration is the stuff of nightmares. The main job of a NAS is to keep the data safe. A file system migration that works in every one of those corner cases present in the wild is statistically unlikely. The kind of bad publicity this can bring is what can sink a company. The only way I'd ever do this is by starting fresh on different storage and replicating the data.
Joe blow running a beta release on his raspberry pi complaining about ram usage isn’t indicative of reality.