-Lose the 4" reducers on your fan. I'll bet those alone are bringing you under 150 cfm.
- Much bigger intake- like maybe 8" to match your 6" fan. 4 two-inch intakes do not equal one eight inch intake- you want a total of twice the surface area in intake that you have for exhaust. Maybe a little more if you want to put in 90's to contain light. Each 90' bend in ductwork reduces flow by about 20%.

You have plenty of fan, just too much drag. Upsizing the fan won't help that much.