Ok, so my fix for this problem was as follows:
1: restrict PASV ports in IIS to 5500-5700
2: ENABLE the FTP Helper app on pfSense for the WAN connection used
3: Create a Rule on the LAN to allow 5500-5700 though, as well as 20 & 21
4: Create a NAT rule to forward requests through 21 and 20 to the FTP server
**Passing port 20 though will allow client machines to use Active Mode on their FTP clients, which seems to still be needed for some specific FTP Clients. For the most part, PASV mode will work with this setup.
For me, I am only using WAN1 for FTP, not WAN2. If you are also using WAN2, you should be able to duplicate the rules just created for that interface.