In my experience (reinforced VERY recently as I’m currently in the progress of making some changes to the network ) multi-homing is simply asking for trouble. Let everyone do what they do best - let the servers serve, and leave routing to the routers!
Besides, it’s easier to control/filter/forward if it all passes through one central point.
BTW, I didn’t quite understand what you said: If I had three interfaces on the TrueNAS, wouldn’t I need three different link-local addresses for them? If so, how would I enter that into DNS without risking failures two out of three on each request?
I don’t follow the above. I guess it comes down to network complexity but getting my Mikrotiks and Piholes to take care of DNS and DHCP seems worlds easier in IP4 than IP6.
I don’t allow IP6 traffic on my network due to the added complexities associated with IP6 firewalling. I read about it once, decided it wasn’t worth the trouble for the time being and turned off IP6.
Every device here and at remote sites has its unique FQDN, IP4 address, and stuff just works, including firing off jobs to remote IP4 printers. To each his or her own, I guess.
It’s also entirely possible that other network stacks (ubiquiti, Cisco, whoever) make IP6 internal and external routing / firewalling / etc. implementation much easier. I’m sticking with MikroTik for now because it’s secure, stable, and does what I need without a fuss at a price point i like.
Just a simple comparison on how to get a static IPv4 and how to get a static (none routable) IPv6. Way easier and smoother for IPv6. Hence why your printer probably uses it by default.
To me the opposite. On OPNsense, IPv6 works out of the box (thanks RA), while for IPv4 I need a DHCP so devices even can get an IP.
I don’t know what should be more complicated about firewalling in IPv6 than in IPv4.
On the contrary, you don’t have to bother with NAT.
But yeah, since not everybody supports IPv6 yet, you basically have to go with dual stack for anything public, which in return results in you now handling both. I think it is fun, but I get if it is not for anyone.
IMHO TrueNAS and other appliances (Proxmox, Loxone) come with very obscure network configurations. I don’t get the strange fixation on static IPv4, strong focus on IPv4, DHCP only on one interface, no privacy extension enabled by default and so on. I don’t really understand why they don’t just simply use interfaces or netplan.
Either way, this is getting really off topic, since OP wants to run IPv6.
Just to make this clear: I am not in the “nobody needs IPv6 camp”. Au contraire - my company is running data centres 100% dual stack for more than a decade.
I am only arguing the DNS and fixed addresses part for clients. SLAAC, default gateway, get out to resources on the Internet - done.
I run almost all my services through a single reverse proxy for IPv4 and IPv6 including SSL termination and since I need to configure the backend manually, anyway, I prefer IPv4 for that. FreeBSD jails, TrueNAS CE apps, VMs … they all have IPv6 for egress, but I use IPv4 for ingress, almost exclusively. Apart from that proxy which most of my clients contact via IPv6, of course due to happy eyeballs.
I don’t see a static DHCP reservation as an extra burden. Central location - way better than manually configuring memorisable link local addresses on each single device.
I don’t bother with DNS e.g. reverse mapping for IPv4 for clients, either.
Oh, now I get it: The link-local address is only unique together with its scope id anyway, so it’s no problem to have the same IP on more than one interface, right?
Maybe OPNsense defaults make this painless, as does UFW on Linux. They take care of ICMPv6 requirements out of the box, otherwise I can see there being some pain to get RA working correctly without opening up too much.
) multi-homing is simply asking for trouble. Let everyone do what they do best - let the servers serve, and leave routing to the routers! 
