Thanks for the explanation @wolke. That is some cool stuff. Will do for sure but i realize i derail your thread into things not related to your original topic.

In that regard it would be most interesting and useful to not only listen from one source $INPUT_DEV but multiple ones.

On Pro¹ we have
/dev/input/by-path/platform-soc\:gpio_keys-event
For volume up, cam-button and Kbd-state

/dev/input/by-path/platform-c17a000.i2c-event-kbd
For the keys on the hwkbd

/dev/input/event0
For volume down and the power button.

How to combine those sources in one script to have all keys available to assign actions too?