Adding IPv6 would cost them money. Probably a relatively small amount of money, but still money. They get nothing from that investment. As long as they have IPv4 addresses to assign to their customers, there's basically no demand for IPv6 addresses. NAT and UPnP work fine for just about everyone.
I think the only way we see serious IPv6 adoption in North America and Europe is government mandates.
Cloudflare tunnel (aka a reverse proxy, like ngrok) will also likely work for your mystery project, and it’s free. VPN is more secure, but as always, it's a trade-off between the security of a vpn and the convinence of a reverse proxy that's available on the open internet.
Basically, I want to move files between my NAS (behind CGNAT) and webserver and rsync isn't cutting it. I think WireGuard will be best, then I can use my existing NFS and Kerberos infrastructure.
Do you need a static IP or could you get away with using dynamic DNS like duckdns? I think wireguard allows you to use a hostname instead of IP address. The wireguard peers would have static private IPs in the VPN address space. I had a much simpler setup than you, but this is what I was doing before tailscale.
A dynamic IP would work; I just need an IP that is unique to my router and isn't shared by a dozen other households---I don't know what the term for that is.
There is a way to make it work with WireGuard using something called MASQUERADE, I'm learning.