Not my area, but what I can say is that the DNS provider I've used for ages (Domeneshop) does not support RFC 2136.

So I guess I either have to switch (to who?) or I set up some alias and run my own bind (another service to admin, yay). Or am I missing something?

I run my own copy of bind, and use https://dns.he.net as a slave pulling the zones from it. As a result I can actually use all DNS features.

I find it super frustrating that most dynamic DNS providers use their own custom API rather than just using the update mechanism that's in the protocol itself. That said, if they support v6 then you can probably use them by installing their update client on each machine that you want a hostname for. It just won't integrate with pfsense.