I agree with Blue. This is pretty much what DMZs are made for.
Another thing you could do is.
Created the DMZ.
Put all your devices on the DMZ interface then make a policy to block PFSense Web UI on the DMZ. (best to put web ui on a custom port and just block that port on the DMZ)
This should block PFSense Web UI from the DMZ side but with rules, you should be able to allow it on the local LAN only, at which point I'd do as you laid out earlier and create a management interface for that traffic.
Another option would be leave it enabled but force HTTPS and change the port number to something totally out of the norm. While it would still be enabled. It would be very difficult for someone to figure out what port it is on and pull it up.
Just a thought.