Well, it’s more complicated, but in theory you could do some deep packet inspection that understands the protocols: personally, I’d use this to break DoH connections (for every host name seen in SNI, attempt a DoH query, if it resolves, reset the connection) and attempt to force everything to fall back to plain DNS. Then, whitelist a couple outbound ports (on most networks, maybe just 443 + 53?) and block VPNs.