Find a contractor well versed in Man. D,check static ,before and after various fittings,the difference is the "drop" thru the fitting,this will show where the largest increases in static are.
If you already have turning vanes ,it may be that the duct system is just to small.The equipment may be larger then needed,and a smaller system might be the best solution.
1244 sq. in of return grille is a lot,using the same filter type,or a little less restrictive might solve the problem.
One thing to remember is that,you are likely getting less then required cfms ,due to the static,so if for example you used a different filter,say .10 PD,the cfms will increase,and so will the static readings in return and supply,due to moving more air.
If you have temperature differences in some rooms,a Man. J should be done to determine the cfm(airflow ) needed in each room,for redesigning the ducts.