styrofoam is already insanely light. It's the lightest possible insulation there is for a garage door. It sounds like your springs are incorrect size range for the weight of the garage door, and springs can't be overtightened, they need to be replaced with a proper spring. Do you own a bathroom scale? Disconnect your garage door opener and set the scale under the door and see how much weight is on the door, it should be 8-10 lbs.

If it’s styrofoam we are talking a couple pounds. If that’s going to stress the door and opener then they are garbage. There is a process to insulate non insulated doors, granted it’s no where near as good as a real insulated door, there are kits and it’s a thing.

If the company though holding the warranty hostage over it then reality didn’t really matter but what the service provider wants. I assume they want to sell you something.