Sorry, what I was trying to say is: you know each app tries to use a certain DNS server. So, in your Rasp Pi, you route their DNS server to point your own (as you would with an /etc/hosts file), that way when DoH occurs, you control the final resolution.
What I'm suggesting isn't merely setting up the 'default' dns server. What I'm suggesting is 'cnaming' the name servers that apps attempt connecting to, to point elsewhere.