This is a trick that helps add life to many plastic products. Each time you remove and replace screws in plastic, they cut new threads. This can be done numerous times before an issue arises, but eventually, it will.
The trick is simple, lightly (and slowly) press down whilst turning the screws counter-clockwise. Eventually, you will hear and feel a ‘pop’ as the screw finds its old threads. It will then thread in very easily.
A handheld manual screwdriver, the type you’ve got on the garage pegboard, will do. This will reduce the risk of stripping out the screws nearly 100%, and you can disassemble and reassemble the valve without concern.