> nextdns.io does a really good job of running such a service.

It is not trivial to build, run, and operate a nextdns-esque service. The founders, Olivier and Romain, are some of the world's best at what they do, imo.

That said, a client-side DNS re-write solution might be relatively cheaper to implement and might even be a tad-easier than deploying an global DNS resolver with per-user custom configurations and endpoints.