Well I understand that the problem w.r.t. a NAS is "simpler" f.s.v.o. that word, but the fundamental reasons behind IPv6's laggy deployment more than two decades after the first "real world" deployments remain. This surely impacts the emphasis, or, rather, lack of emphasis on IPv6 in many products, including TrueNAS.
I know from direct experience that FreeBSD interactions between the bridge, epair, multicast, and ipfw are a real PITA to investigate and resolve. IPv6's heavier reliance on multicast (no broadcast address) tends to tease out certain firewall rule issues and other fun stuff, and with FreeBSD's twitchiness about multicast address assignments on bridges, I can see that being a world of pain to get all the finicky details just right.
As for iocage, I thought that was strictly a userland jail manager.