Based on the hardware you have, if you’re able to install it on one of the routers, I know that OpenWRT can set DNS on a per device basis with it’s DHCP server.
My personal suggestion is to use a PFSense device as there is an option for per device DNS settings, along with a lot of other options that you might want to mess with if your trying to learn some networking
Pi-hole setup help
First time posting here, so please lmk if I should format this question differently or ask for this kind of help somewhere else....