I read up that later Instars (5th growth stage) will some times eat watermelon and/or pumpkin.

I'll bet there's other people growing milkweed in your area. Have you tried checking around locally? I used to use wild milkweed also. Unfortunately, Monarchs are not seen much around here anymore. I only had one caterpillar that actually became a butterfly this spring. Maybe I'll see some more this fall.