Wait a minute... First Google DNS provide both DNS-over-HTTPS and DNS-over-TLS, second Pihole (or should I say dnsmasq, or FTL the name of their dnsmasq fork) does not support forwarding DNS query request to upstream using neither DNS-over-HTTPS and DNS-over-TLS.

In any case, if you really want a full solution, build your own https://github.com/yegle/your-dns