Thank you for your summary.
Things are indeed probably more complicated than we think. But what @ask had in mind, seems overly complicated to me. As he’s most likely busy enough as it is, with keeping the NTP pool operationally stable, it is probably safe to cut a few corners and simplify his initial plans with regard to IPv6, especially because a lot has changed over the years and IPv6 is now much more common than it used to be.
If it was up to me, I wouldn’t make IPv6 optional (in vendor zones - item 2). Just treat IPv6 the same as IPv4 in the entire pool.
With regard to item 5; some 58% of all participating servers globally, are IPv6 enabled (which is sufficient to handle the anticipated load) and a quick glance shows a similar distribution per region.
Even on a per country basis the figures seem to be looking good, based on some random samples. So I wouldn’t worry too much about this.
If IPv6 is enabled gradually, with sufficient communication towards server operators (and vendors), things should work out fine in my opinion.