As recommended by others, you might want to use a real public domain that you own, and a reverse proxy for split horizon DNS. I personally run Bind9, Unbound, and PiHole as my DNS servers. Bind9 handles split horizon, so if I request my domain internally it gets routed through Bind. Then bind hands it off to PiHole for adblocking, and PiHole makes requests through Unbound set up as a recursive DNS server which doesn’t rely on any external DNS. I also use Traefik as a reverse proxy for all of my services. My set up is more complicated than necessary, and if you want just a few local domains, PiHole + a reverse proxy is plenty good for your needs.