Will your fan have a louver or any other accessories? I would call up a fan representative and ask for a recommendation. Otherwise for a wall exhaust fan typically I would specify 1/8" static. Good luck.
Use SMACNA round industrial duct construction manual and then talk to a fabricator about which gauge he would use. A heavier gauge may be easier to fabricate and might require less reinforcement.
Can you manually run the fan on the air handler? If so wouldn't that cut the humidity even if the thermostat wasn't calling for cooling? I remember something about this from school but maybe I'm wrong.
You should not be adjusting dampers in a dust control system after the system is balanced generally speaking. Any damper adjustment will affect the system balance and the carrying velocities in the main duct.
A VFD is a good thing to have on a system that includes a baghouse or dust collector...