I don’t know if this is helpful to you, but I’ve managed to run scripts after the xsession is up and running as described here: USB Touchscreen input scaling issue on 7" 1024x600; I'm almost resolved. One issue left! - #4 by musicToMyFears
I would be quite interested to see your project in action. I own an Elgato StreamDeck which sat on my desk collecting dust for a long time, and now sits in a box to keep the dust off. This would be a great use for it!