zfs-unlock

#!/bin/sh

# Place this in /etc/initramfs-tools/scripts/init-premount/zfs-unlock and
# run update-initramfs -u to include the script.
#
# This initramfs script handles mounting encrypted pools from a keyfile
# and not a passphrase. The main use case is keeping datasets encrypted so
# disks can be safely discarded if they partially fail and can't be easily
# wiped.
#
# The keyfile is placed on a USB key with the label KEYS, and is unmounted
# when done.
#
# root-on-zfs hasn't been tested yet.

# Change to the name of your pool.
POOL="pool"

PREREQ="udev zfs"
prereqs()
{
   echo "$PREREQ"
}

case $1 in
prereqs)
   prereqs
   exit 0
   ;;
esac

. /scripts/functions
. /etc/zfs/zfs-functions

exec >/run/initramfs/initramfs-zfs.debug 2>&1
set -x

# Custom content starts here.
# ------------
# Source the default setup variables.
[ -r '/etc/default/zfs' ] && . /etc/default/zfs

# ------------
# Support debug option
if grep -qiE '(^|[^\\](\\\\)* )(zfs_debug|zfs\.debug|zfsdebug)=(on|yes|1)( |$)' /proc/cmdline
then
	ZFS_DEBUG=1
	mkdir /var/log
	exec 2> /var/log/boot.debug
	set -x
fi

# ------------
# Load ZFS module etc.
if ! load_module_initrd; then
	disable_plymouth
	echo ""
	echo "Failed to load ZFS modules."
	/sbin/modprobe zfs || panic "modprobe zfs failed. Manually load the modules and exit."
fi

zfs_log_begin_msg "Mounting keys."
mkdir -p /media/keys
mount -o ro,umask=0377 /dev/disk/by-label/KEYS /media/keys
zfs_log_end_msg

zfs_log_begin_msg "Loading keys and pools..."
# Don't mount file systems to prevent locks when we pivot root.
zpool import -N $POOL || panic "zpool import failed."
zfs load-key -a || panic "zfs load-key import failed."
zfs_log_end_msg

umount /media/keys

This was on Ubuntu 18.04 - I haven't upgraded that host to 20.04 yet. I also don't have root on zfs as I was waiting to upgrade to 20.04 to do that. That installer looks great, thanks for the link!

My suggestion would be to install first with a normal interactive passphrase, and then iterate on the above. Worst case is you have to do attended (re)boots until it's working. On 18.04 the original version of this script is at /usr/share/initramfs-tools/scripts/zfs so it may be worth comparing that for any changes.

Yes it's a good alternative installer, you are welcome.
Thank you for your response and sorry for the late reply, I'll try your solution and iterate on it.
If I find a working solution I'll add a link here for reference.