You could, but DoH exists to provide a crypto-based guarantee that they can't even passively monitor the DNS requests you're sending to the alt. resolver.
DoH only hides the domain name lookup. You still connect to the other server by IP. Even $2.50/mo VPS plans offer a unique IP these days, and building a database of IP:hostname is extremely trivial, especially for ISPs that already run DNS resolvers.
But let's be real: the internet is quickly becoming a walled garden, so having access to DNS requests is mostly only going to give you a billion facebook.com + twitter.com + youtube.com + google.com + google-analytics.com lookups anyway.
But let's be real: the internet is quickly becoming a walled garden, so having access to DNS requests is mostly only going to give you a billion facebook.com + twitter.com + youtube.com + google.com + google-analytics.com lookups anyway.