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A helicoil or a threaded insert will be quite strong. The difference is that the helicoil tap cuts new threads. It is commonly used in castings. An insert is often bonded to sheet materials. The trick is finding the insert that is the same length as the thickness of the drop out. I take it the frame is aluminum? If you go with a machine screw mounted to the drop out use a fine thread pan head screw and a jam nut then mount the derailleur with a nylon insert nut or use blue Loctite thread sealer.
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