It's the inherent problem of your network position being between a device and the outside internet, the same position that browser activity-selling, website blocking ISPs are. The only difference is that previously, DNS would be unencrypted so you could DPI dns and/or block DNS requests going outside of your custom resolver. If an attacker wanted to hide DNS previously, they would need a hard-coded IP address to send encrypted requests to; now they can use big providers like CF/Google for DoH.