Aren't there serveral hosts now where IPv6 access is included but you have to pay for each attached IPv4? E.g. AWS and Hetzner

Yep

just a few hours ago we found a pretty nice residential desktop use case for proper v6 (with prefix delegation), due to no need for NAT the old router (2013) became less of a bottleneck!

Double check (I mean by remote port scan) that your firewall is working. I’ve seen routers with no IPv6 firewall . And it actually matters