I also recently bought a large printer, a CR-10 S5, and I haven't printed with it yet but what you're saying is very concerning.

Maybe you can change the glass bed on your printer? Event if the aluminum plate that it rests on is bent at the factory a new glass bed on top of it should be straight. You can get a glass bed at a local hardware store for cheap.

If the new glass gets bent then place thin metal strips around the the aluminum plate to straighten out the sides of the glass bed.