Can you do an ipv6 address behind a DNS entry and have your friends use the DNS entry? Assuming you can get a cheap domain name. It's a little bit more complex but it sounds like you know what you are doing. Then just allow the ipv6 traffic through your router (assuming you and your friends get ipv6 through your ISP) another option is a VPN, it will add some overhead but it would be better than nothing.
Or if you are feeling brave duel boot into a Linux distro like Ubuntu and try running the Linux version of the server 🤷♂️ its pretty easy to do once you get used to Linux
Eco servers don't support IPv6 - you need to have IPv4.