This doesn't work because if you run your own recursive DNS server, it will make insecure requests to all of the authoritative servers, and so your ISP can hijack them all. And DNSSEC will keep you from getting sent to the wrong domain, but won't help you figure out the right domain.
Your private recursive DNS server, of course, has to send requests to the rest of the internet; you don't want to block those. They don't have to be plaintext, unless the authoritative server in question only talks plaintext.
The point is that these requests don't go to my ISP's DNS servers.
And for the other people who're making up unrealistic scenarios such as the ISP trying to MITM all DNS, not just queries they answer, there are many forms of tunneling that can be used such as VPNs. It's still easier to do one solution for the whole network than individual solutions for each client (or each application, for DoH).
Moving DNS from an ISP, who we pay and with whom we have legal contracts, to a company that does things, supposedly, for altruistic reasons, with whom we do NOT have contracts, doesn't fix anything. It makes things worse. The solution is to remove DNS from your ISP and run it yourself, or use a not-for-profit that isn't trying to become a monopoly, that isn't in a position to have its data syphoned off by the NSA, that doesn't knowingly and willingly host spammers, phishers and scammers.
How about we don't trust ISPs AND we don't trust Cloudflare?
BTW - I have to flatly disagree with your suggestion that, "once you've got one MITM it becomes harder to trust that there isn't a second one". That's ridiculous. I can check and verify things to a much greater degree by running my own network. Also, I never said anything about MITM my own network. I want to run my own DNS and block DNS to the rest of the Internet. That's not MITM.
The least wrong thing is to not replace something that MIGHT be shitty with something that MIGHT also be shitty, but might also open you to new problems and security issues. The idea that it MIGHT be less shitty isn't a good enough reason for DoH.