For rather cheap I can see what traffic is suspicious. If you throw more resources at the problem and scale up it becomes simple to see traffic that looks like dns over https without having to decrypt it. Indicators such as size, frequency, consistent traffic going from your host to your DoH provider and then traffic going to other parts of the internet….these patterns become easy to establish. Once you have a good idea that a host on the internet is a DoH provider you can drop it into that category and block it.