Another quick update:
I got the boot pool to boot without hanging in initramfs by clearing the drives before install with:
blkdiscard /dev/sdh -f
My previous attempts using
zpool labelclear -f /dev/sdh || true
sgdisk --zap-all /dev/sdh
and the formating done by the installer itself is not enough as it seems. Something must have been left on the disks that caused confusion.
@HoneyBadger Maybe that is interesting for the dev team to bake this into the installer?
Seems to have been an issue before: