In terms of responsiveness, you’re probably better off fitting the 12TB on a single 14TB drive attached to a NUC8i7 running ROCK, and mirroring that to a NAS for backup purposes.
If you absolutely want a one box solution, and are comfortable with rolling your own, you might want to have a look at UnRaid, though it’s likely overkill.
Given the choice, I’d personally pick the NUC + NAS-for-backups and get true data redundancy. Assuming you’re comfortable with a screwdriver, and you want to migrate a NUC + 3,5" to an ok looking passive case, the HDPlex H1 V3 should have you covered at some point.