After searching around a bit more on my own, I found this other post you had commented on: Pihole-ftl using all my CPU and breaks all internet connectivity - #6 by jfb.
I went ahead and followed the mv instructions there and restarted FTL - I am going to forward my router back over to point to PiHole and monitor - see if the same traffic shows up again like before.
I did run a pcap on my router and didn't see the issue traffic there this afternoon - but will continue to monitor.