I would install more trusses between the existing trusses. Make my own trusses using stick boards, a jig and water (to bend the boards in a convex shape). Then, install a few days later. I'd use deck screws and galvanized fasteners. . Ya, lots of individual board and custom manual work. But, well worth it in the long run. Especially if you want to remove the soft feeling from the roof. I would then install (glue and screw) 1/4" plywood the the new roof. With less distance between trusses, 1/4" plywood is good.
After plywood is down and its but joints are filled / sanded, I'm probably lay down some liquid EDPM (instead of sheets of EDPM). Liquid EDPM is bright white and best of all, it it doesn't shed its outer layer - to create black streaks (like sheets of EDPM). For more info, surf:
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