As silly and astonishing as it is, I've heard from some (mostly American) ISPs that a static IPv6 subnet is either not available for consumers or costs extra.

Yes, that's right, some ISPs rotate IPv6 subnets, negating many things IPv6 was invented for in the first place.

Tailscale, Nebula or any of the automagical VPN solutions you can run yourself (like Innernet, https://github.com/tonarino/innernet) will probably negate the issue as long as you can reach some server with a static IP.

Of course, because if you can use it to offer services, it’s a ‘pro’ connection!

Do other people use dynamic DNS services offered by domain registrars? I've use one for years and never have had an issue.

I've never needed to. I don't think I've ever been with a non-mobile consumer ISP that rotated IPs enough for it to mayyer. Maybe if I took the modem offline for more than a week or if I move, but that's the only times I've ever had to update the necessary IP addresses.

It's still not a great fix, of course, because DDNS still causes outages while intermediate servers wait for their TTLs to expire and caches to clear, which means your record could point to the wrong IP for at least one minute per switch. That's fine for a mail server, but not great for other applications that don't handle servers dropping from the network so we'll.