Is there any security risk to removing the existing DNS server addresses on my router? Sorry, I'm not the best with network engineering...
Short answer, no it won't decrease your security. Meddling with upnp and or firewall settings on the router may compromise security. Plus you can use on pi-hole additional features like dnnsec and (thrid-party) malware/botnet block-lists so hat you have authentic dns lookups and avoid bad traffic initiated by malware on computers and phones -- to a certain extent.
Ok, I'll do that soon. One last thing I forgot to mention; I made a wildcard for googleadservices.com and the ads still aren't blocked... It's as if they're forcing their way through...