Unfortunately the monkey's paw would curl here and using DNS other than that provided by your ISP would become illegal.

(I know there's a difference between your devices using their network to access the internet and your "purchased" (licensed software running on) your device using your network to access the internet - but the monkey doesn't care)

Counter point on that, that might affect the legality of DNS over HTTPS

RFC 2132 says DHCP option 6 provides "a list of Domain Name System name servers available to the client". That's a MAY use, not a MUST use. You are welcome to block DNS requests to outside servers if you prefer.