And if all else fails, you can put something like Cloudflare in front of it to handle IPv4 traffic.

Might be a good idea to make a hidden request from your homepage to an ipv6 only resource. Then log how many of your visitors actually fail.

It's a chicken and egg problem: as long as sites are available through ip4, ISPs have no incentive to provide ip6, and since ISPs often don't provide ip6, sites can't go ip6-only. One possible solution would be to provide both and throttle ip4 traffic, then better speed can provide incentive to upgrade to ip6.