Been using afraid.org for well over 10 years and use dynamic dns to have various subdomains pointing to different IP addresses/hosts I have in physically different places. It just works and I login maybe once every 3-4 years.

I'd second afraid.org, have been using them for years and they've always been great. They also support dynamic DNS so if you're on a dynamic IP address you can have the address be updated automatically when your IP address does.

More relevant to the question, I'm pretty sure you can create NS records for a subdomain as well. I was experimenting once a few years back with a DNS tunnel service and was able to get the DNS side of it configured. Never did get the service itself working but it was more of a curiosity at the time so didn't spend a massive amount of time on it.

Decent DNS providers allow you to create NS records for subdomains.
This delegates the subdomain and all of its subdomains to another DNS.

Useful for companies that want to control their own records, but might want to allow a group of developers control over app.example.com and all subdomains, without the developers having to pester the company for record updates.

Also used for acme-dns, which is a self hosted DNS designed to only deal with txt records for acme DNS challenges (ie lets encrypt).
Means you can limit the possible disaster of the DN API keys being leaked (an attacker can only generate TXT records, instead of rewriting all your DNS records)