I think I identified and corrected both of those issues. The random Google DNS entries were coming from an Android emulator I had running, and they went away as soon as I closed it. I previously had my pi-hole configured with a static IP address from when my ISP delegated an IPv6 address to me, but I only have an IPv4 external address currently (I'm guessing this caused my pi-hole to auto-generate a new ULA address when my IPv6 address changed). I updated my pi-hole's IPv6 address to use a ULA address within OpenWrt's LAN address range.
Ignoring the erroneous Google DNS entries, all my desktop's DNS traffic is between my desktop and my pi-hole, in spite of the DNS servers listed in ipconfig not updating to reflect what's actually happening on the network.
