You can set up firewall rules to redirect the traffic destined for public DNS servers to your internal DNS server.

Not sure how to construct that rule in the unifi firewall but it comes down to "any outbound traffic on port 53 that's not destined for the adguard server, redirect it."

You don't need to be home for a cron job to run.

USB has a bad habit of randomly dropping off the bus until you reseat the cable or reset the device.