Disclaimer: I have zero experience with zfs and I haven't installed FreeBSD in a while.
However, I do know that to boot in UEFI mode you will need a FAT-formatted
EFI system partition (type "0xEF" in the old MBR nomenclature) with FreeBSD's EFI loader placed at $ESP/EFI/BOOT/BOOTX64.EFI — any UEFI firmware should boot that loader automatically if it is found at that location.