> if your users don't have IPv6 (and many don't), you're basically screwed
Not quite, all you need is to make sure the proxy/CDN is dual stack, like what this IPv6-only hosting service did: https://www.mythic-beasts.com/order/rpi/
Cloudflare, for example, does this for free: https://community.cloudflare.com/t/proxy-ipv4-visitors-to-ip...