The screen flickering is a commonly reported Ren'py issue which tends to happen on newer computers/laptops. I'm not 100% sure what causes it, but it's a problem with Ren'py itself, not the game.
If you encounter this issue with any other Ren'py VNs, you can check this post here for some advice on how to fix it.
I'm glad you enjoyed the VN other than that, though!