A user has found a way to get adb access to the Shelly Wall displays by soldering wires to the RX/TX pins directly and using a UART-USB adapter: Shelly Wall Display - #16 by zittix
Sadly I can’t find these pins on the TPP001-Z PCB we’re having issues with, as nothing has labellings… I tried to inspect it a bit, but had no luck identifying anything. I know which pins of the BGA of the CPU are the UART pins, but that sadly doesn’t really help 
Page 17 and following: https://rockchip.fr/RK3308%20datasheet%20V1.1.pdf
If somebody else has a smart idea on how to find (potential) RX/TX test points on the PCB: Would be worth a try!
I’ve updated my post above with pics from the PCB with the heat cover removed and the back side: Tuya / Smatek "T6E" and ”S6E” 4-inch Control Panels for CCS (Central Control System) Android-based in-wall 4-inch smart display with touch screen now sold under many different brands - #61 by ezcGman