arch linux on zfs root cannot configure grub on bios
I'm not sure about arch, but I think this:
zfs=rpool/ROOT/default
should be:
root=ZFS=rpool/ROOT/default
Are you sure the arch build of grub supports zfs? Did you make sure the ZFS pool was created without any of the feature flags that grub doesn't support?
On EL7 /boot on ZFS was easy enough, but on EL8, various questionable grub configuration changes made it impractical, so I gave up on that and just have a small mdraid partition for /boot on xfs. It's not perfect but it's livable with. You may have to do similar in arch.