Absolutely. This makes for good discussion.
Strictly on the FTTH end of things to make it simpler (since that's where our growth is). We're very much residential with the commercial customers being farmers, home based mechanics/construction/horse stables, and the like - less Netflix and more websites.
1. Our ONT (gpon+nat+wifi) I'm all but certain would work out of the box. Now if the customer wants their own router in which case I guess we force them into one methodology for v6 (correct me if I'm wrong but there isn't a standard way to deploy v6 globally like DHCP, it needs RA which leads into obvious issues with the kid's 14 antenna Netgear).
2. Calix supports it (OLT), I suppose we would just need to enable v6 on the DNS servers and we're done
3. I don't believe our software supports it, but I'm not certain, obviously I haven't looked
The biggest issue you completely left out is operational. I'd have to spend time adding on v6, which is a bit of a time sink but at least there's a nearby finish line. Time I'm not getting fiber hooked up to houses. Now when (not if, we all know nothing is perfect) there are issues with v6 but not v4 I have to figure out why "my internet is slow" when v6 has problems. Setting something up once, be it fiber or v6, is a one time capex. Ongoing support problems are indefinite opex.