> Just trying to set it up on a single USB drive

That's the fault of macOS, I also experienced 100% CPU and load off the charts and it was kernel_task jammed up by USB. Once I used a Thunderbolt enclosure it started to be sane. This experience was the same across multiple non-Apple filesystems as I was trying a bunch to see which one was the best at cross-os compatibility

Also, separately, ZFS says "don't run ZFS on USB". I didn't have problems with it, but I knew I was rolling the dice