You most likely have created a DNS forwarding loop somewhere causing high numbers of queries because queries got trapped in a loop.
Do you have conditional forwarding (CF) enabled in Pi-hole ?
Try disable this one for diagnosing.
And tell us what IP is configured in the router WAN/Internet DNS settings, is it that of Pi-hole thus potentially creating a loop if CF is active ?
EDIT: Try default all your router DNS settings and do exactly below to distribute the Pi-hole IP as single DNS server to all your clients through DHCP: