Remember that Apple likes to be exempt from societal and digital norms…
You should probably do the DNS blocking on a network level, and not on the machine level. It is sort of useless to wrestle with the underlying OS when the developers of said OS is actively making DNS blocking difficult for you. All your efforts can be nullified after their next patch cycle.
Get a PiHole or a pfSense+pfBlockerNg or something like what ProtonVPN does with a VPN- level DNS filtering.