DoTLS and DoHTTPS can be intercepted on your own devices by adding a privately generated certificate to your root store and MITMing the traffic. By design this is the only way to filter this traffic based on content.

Regardless name resolution based ad blocking is relatively futile against even naive workarounds. For instance what is to stop someone from using a custom DoHTTPS format on any webpage to resolve the name directly in browser? What's to prevent them from obfuscating it in a way your MITM couldn't realistically detect?

In the end ad blocking is best done directly on the client through something like uBlock Origin. Not only does this allow you to create a network request block list (now with the added capability of reading/filtering on the whole URI) but it also allows for style based blocks where the ad content could even be blocked if it comes from the same server and resources serving the actual page.