Opnsense is based on HardenedBSD. Different from FreeBSD (which pfsense is based on).Try some of the suggestions offered here: https://forum.netgate.com/topic/58160/root-mount-error/8Even though it is for FreeBSD some of the suggestion might be applicable.
Comparing to 2.4.5-p1 ISO the file is only 372 MB. I'm guessing you are using a similarly slim image for FreeBSD? Are you sure your disk and drive and target system support DVD size?Cheers,Franco
have you tried the method I posted using opnsense-bootstrap?
IMHO There's little point in using ZFS on a single drive. Leave ZFS out of the equation and try again.