Pi-hole v5.0 should not force itself as the resolver. The crash that is reported in your debug log is identical to the one see here, undermining some "real" bug:
Could you check if you're on the most recent version? What is the content of your /etc/init.d/pihole-FTL identical to this one?
https://github.com/pi-hole/pi-hole/blob/release/v5.0/advanced/Templates/pihole-FTL.service
Specifically, we're looking for the absence of /sbin/resolvconf in this file: