I am not an expert in the domain of DDNS. Wanted to bring your attention to desec.io, in case you didn't knew about them. They offer a similar feature set like you mentioned (IPv6, DNSSEC, BYOD, ...). It is an open source project and they offer a very reliable free hosted service. As you said, they originated from the 2010-era (2014). I've used them for several years now and they bring everything to the table that I need.
For inspiration: They even have a feature that I use which I haven't spotted in your documentation (but maybe I just didn't looked close enough): Support for IPv6 prefix delegation. Routers that get assigned an IPv6 prefix from the ISP, can update the IPv6 prefix of arbitrary domains. In Europe this prefix is not static and rotated each time a new connection to the ISP is established. This feature allows the router to automatically update the IPv6 _prefix_ of selected domains. The host part of the IP is left untouched, but the network part is updated.
e.g.: /update?myipv6:nas.home.mydomain.tld=2003:e6:bee:affe::/56