This should be possible but I haven’t tried it myself for a few years so I’m not sure if it still works as expected (it should, though).
Create a file
/etc/dnsmasq.d/99-custom.conf and put there:
/etc/resolv.conf should still only be:
Then ensure that the servers are in the correct order in
/etc/dnsmasq.d/01-pihole.conf (first listed = first asked) and run
However, I haven’t found it necessary to define a fallback solution for Unbound which is working 100% perfect for me since years in a mid-business sizes environment.