If I can't tell, neither can the legitimate users. This makes queries that would out of the box rather hard.
And you still have the traffic patterns that will give you the information with sufficient confidence.
And you still have the traffic patterns that will give you the information with sufficient confidence.
And blocking https packets with mere confidence is impossible.
So, yeah, I can serve up DOH on my own server with a custom endpoint, custom client configuration, and generate cover traffic. But that's not exactly easy. And http doesn't help: I can do that without http, something like https://github.com/yrutschle/sslh, or using SNI (which, as of TLS 1.3, is encrypted).
And merely saying something is impossible doesn't make it so. It doesn't even make it hard.