Self-hosted DDNS as part of a consumer product offering (potentially >>1000 sub-subdomains), how? [closed]

The assumption that

this relies on a normal port-forward on the user's internet-facing router

will work reliably without “expert” intervention for the majority of your users is probably flawed.

The typical solution https://en.m.wikipedia.org/wiki/Universal_Plug_and_Play is not universally supported at all.

Unless you expect that all your users will always be on “home” networks which allow them to set up port forwarding manually when it doesn’t work automatically you should really rethink your design.

In general you see that clients make use of a central server / service (over https on the default https port) rather than peer to peer connections and then the you don’t need to assign dns names to find your users. Your users will find you (or rather your servers)

which makes your question and intended solution a path you simply shouldn’t take.