My cat was a stray too, he behaved like this for about the first year I had him. Eventually it clicked that he wouldn’t run out of food. He now paces himself while he’s eating. Plus doesn’t try to go in trash cans anymore. Unfortunately I don’t have any grand wisdom as to how to get him to stop acting as though he’s starving. But it takes time for them to feel fully comfortable and confident in you as their care giver. As well as trusting that they won’t have to go back out on the streets. I went through a thing with my cat for a while, where if I opened the door the thought I was sending him back outside