Have a look to internal/external bridging: https://docs.netgate.com/pfsense/en/latest/book/bridging/index.html
You just neet that one IP inside your network? I don’t know your exact topology and setup. Buf if you have a public IP block, your ISP can route that to you via a small /29 subnet or smaler (/29 is minimum for HA setup). You define that /29 subnet to your WAN interface and use your public block inside your network just like a private network.