By default, Ren’Py shaders are applied to individual images, using the image’s relative coordinates. To fully apply this to a layer, I’d recommend showing a Layer() displayable with the CRT shader applied. The “mesh True” transform property could be of help in this scenario.
If you’re in either the DevTalk discord server or the official Ren’Py Discord, we can continue there (with the results being posted here afterwards)