You can DNAT outbound port 53 connections to an internal server. Any router/firewall with configurable NAT can do this. This is a must with some smart TVs for example.

My Mikrotik router does this easily. You can tell it (with a Firewall NAT rule) "any outbound connection to port 53 is to be redirected to this internal IP and port" -- and this internal IP and port is where my PiHole is.

Yup, with the Adblock package on openwrt this was a one click option in the GUI. Doesn’t help with DoH unfortunately, but it definitely helps in general.