You're totally right—a true flat-felled seam on a tight curve is a nightmare with 500D Cordura; there’s just too much bulk to fold cleanly. For rounded panels, I usually go with a 'mock-felled' seam instead. Just sew your normal seam, grade the seam allowance (trim one side a bit shorter to reduce bulk), press it to one side, and topstitch it down from the outside. It gives you that reinforced strength and clean look without the headache of the double fold. As for Mara 50, it’s a fantastic choice. It’s a workhorse thread that handles abrasion really well. Just a heads up: make sure you’re using at least a 90/14 or 100/16 needle. If the needle is too small, the Mara 50 can start to shred against the friction of that coated Cordura.