Hi, doing on mobile so short answer. To my knowledge they don't do RFC 2146 but rather base everything around a good api that they have. Like you say different types of records etc.

And really, dynip came to be from fortinet/fortigate that have excellent support via their genericDDNS setup and things keep of of grew from there to what you see today.

And the subnet ipv6 sounds really interesting. Will need to check that out, sounds like that could be a feature request

i can vouch for desec.io for having the option to have TXT, NS, CNAME, etc dns entries on their free tier! (limited to 1 domain, up to 50 entries)

i really had a bad time trying to get a letsencrypt certificate through the regular auth because it does require ports 80 and 443 tcp that by ISP blocks.

(you can get a letsencrypt cert through a TXT entry too, but most free DDNS´s providers dont seem to offer that)