The most important part of DoH, etc is that it allows you to make a choice. You can choose a vendor in your country. As a Canadian, I might want to use the service offered by my national TLD operator https://www.cira.ca/en/canadian-shield/configure/firefox/
Many ISPs explicitly sell DNS data, and are also advertising vendors.
With insecure DNS, the choice isn't meaningful since your ISP will see all of the data no matter which DNS server you pick to use. And those kinds of ISPs will probably block DoT because they want to keep seeing it all, but they can't block DoH.
I put my DNS service on a non-standard port. I’m the only one using it so standards be damned. Windows doesn’t allow setting a nonstandard port for DNS, but pretty-much everything else does.
Do ISPs do deep packet inspection to get lookup data? Maybe, but it increases the cost of doing so and makes the business aspect of it less viable. Perhaps a minor win.
With cleartext DNS, your queries may never reach your chosen server. Plenty of ISPs are configured to just answer any DNS query, regardless of its destination. Using a nonstandard port might help, but you’d be much better off deploying one of the DoH / DoT / DoQ / etc secure protocols.
Many ISPs explicitly sell DNS data, and are also advertising vendors.
Cloudflare, on the other hand, doesn’t share or sell data and retains minimal data: https://developers.cloudflare.com/1.1.1.1/privacy/public-dns...