You can do the test by yourself, but many NextDNS users (me included, also with Quad9) noticed that there was almost systematically a DNS leak happening with iCloud Relay turned on. There are many comments on the NextDNS Reddit in that regard. Oddly enough, it doesn’t happen on macOS. Only on iOS.
There’s also a bug that the dev behind NextDNS tried to “fix” (meaning begging) Apple to fix.
And when it comes to ODoH, NextDNS didn’t and won’t implement it. As jonah stated while I was writing this message, ODoH wouldn’t do much anyway.
As far as I know, the guys behind NextDNS really got in touch with Apple to improve things.
Alas, there’s much more they can do besides waiting for Apple to update their priority list…