Just to see if dnsmasq's own cache isn't getting overwhelmed by having too many requests (it defaults to 1500 entries), see this post here on how to monitor cache recycling:
Dnsmasq always runs as the DNS proxy and resolver on Asuswrt, it's not tied to parental control.
Hey Merlin just to follow up, I turned on DNS filtering in parental controls, using the "router" DNS (in my case, OpenDNS) and the queries dropped dramatically... from thousands, to hundreds. I am speculating that this setting somehow forced caching, or that without it the requests were being passed up stream? Not sure if that's expected behavior. I'll keep an eye on it but it's seemingly doing what I wanted in the first place.