You can generate valid certificates for the domains you own and make the DNS point at anything you like. It's quite a pain for a dev setup (LE certificates only lasting three months, so long enough to forget about your setup but short enough that you'll need to keep running it).

In this specific case, it's about, a bunch of generic domains set up by other people.

In your pihole example the situation would be even better because you don't need to publish A records for the domains anywhere. That means nobody can abuse your domain for fingerprinting workarounds but you still maintain complete control.