I think the last line is the major concern for the OP. It's fine and dandy if your browser _lets_ you use different DOH server with it's own self-signed cert. If you're locked down... well, it's game over.
No reason you can't get a real certificate, to ensure that your device talks to the server it intends to.
> If you're locked down... well, it's game over.
Devices today can already use pinned certificates and other similar measures to ensure that they only talk to the server they want to talk to. DoH doesn't make that any more or less effective, just simpler to get right.
You'd have to get a certificate for whatever domain the device wants to talk to - a domain you very likely have no control over.
No sane CA will give you a cert for that. The whole point of the PKI is to keep MITMs from pretending to be domains that they actually don't control - which is exactly what we'd be doing here.
If 90% of users leave it as shipped set to (say) Google, 9% switch to their preferred service and only a handful like you actually type in a DNS name, that freedom to do is preserved. If it doesn't check the certificate from the DoH server when you override the configuration, you are immediately exposed to a MITM so it needs to check. However you'll have no trouble getting a cert for your own FQDN, and that's what the poster you replied to is thinking of.
I believe there are ideas for a network-level autodiscovery of DoH servers, analogous to regular DNS. That would solve the problem, but I can't see how it would get buy-in from the DoH crowd, as it would tie DNS to the network again - and it would still have to be implemented by every single device.
Of course all of this is moot if the vendor actively wants to prevent you from intercepting DNS.