Your machine is not using Pi-hole, have you set the DNS server manually in your machine's settings? If not, then try these commands to refresh your DHCP settings:
I have set the DNS server for both ethernet adapters (I use Evolve for gaming with my friends without port-forwarding) and refreshed the DHCP settings multiple times.
I have the pi-hole (10.0.0.40) and the google DNS server as a backup (8.8.8.8). I don't know what the IPv6 addresses are though, and I don't know exactly how to or if I should remove them.
You should remove all the DNS servers except Pi-hole. If you have more than one DNS server, your computer doesn't have to use them in order (Ads might be resolved through your secondary instead of Pi-hole).
Pi-hole uses that as an upstream DNS server. Your DNS requests go through Pi-hole, get filtered, and non-ad requests go to the upstream servers.
Check the IPv6 adapter settings for IPv6 DNS server settings.
Then set the router to use it and manually set the DNS server on the devices you don't want to use Pi-hole to something else. Or the other way around. Either way, Pi-hole can't be used alongside another DNS server effectively.
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.