Bumping this and adding more specifics...
According a post on the Pi-hole forum, the correct config is:
Add the Pi-Hole IP address to pfSense > Services > DHCP Server > DNS Servers.
Do not enable DNS Forwarder.
Do not enable DNS Resolver.
Do not add a DNS entry in the System > General Setup > DNS Server Settings.
The last setting seems to be causing an issue -- the router is unable to connect to my VPN provider if no entry is made in System > General Setup > DNS Server Settings. It's also unable to connect if the Pi-Hole IP address is entered there.
Specifying a public DNS, such as Clouldflare, does work -- but then I am not sure if all DNS queries are going through the Pi-hole?