Should I change the routing of the return air flex duct?
I think they ran the duct longer to try and avoid it collapsing coming over the edge. To me it looks to be partly collapsed going around the support. To improve the flow I would add a 90 and shorten the flex. This is not hard to do. Get a 90 and 4 zip ties long enough to go around the flex. Use a razor knife to cut the flex and a pair of pliers to cut the wire slip the inner duct on the 90 and zip tie on both sides. Now pull the insulation over the 90 and zip tie in place. If you were able to remove several feet there should be enough insulation to cover the 90. This will eliminate the possibility of the flex pinching off The air flow and with a shorter flex run less resistance so more air flow.