I don't have any resources handily available for that. I would be truly surprised if pi-hole's don't support DoH though so I'd just try searching for something like "enabling DoH on a pihole" or similar.

Basically sounds like you've already done the hardest parts. Your router is redirecting all DNS traffic to your pi-hole, this will prevent any normal unencrypted DNS traffic from leaving your local network. You pi-hole will be in turn making all the DNS requests. If you turn on DoH for the pi-hole all the DNS requests on your network will be encrypted.