That only fixes the "Cloudflare" problem. It still centralize all DNS traffic onto a single service. I cannot configure Firefox to send packets only to the (hypothetically DoH capable) delegated nameserver(s) for each domain.
When you run your own recursive resolver (which is easy), only the lookup of the NS delegations needs to use a centralized service. This is relatively infrequent (NS records rarely use a very short TTL), so regular (TTL-respecting) caching prevents the centralized service from learning your pattern-of-life.
The DNS traffic for the A/AAAA records[1] is very different. Short TTLs are commonly used to force a DNS check every few minutes. In practice, this means a user's aggregate DNS traffic contains a very good pattern-of-life about the services they use and when they use them. When you run your own recursive resolver, nobody can learn the entire pattern-of-life; the traffic is distributed to different servers for each domain delegation.
