How to configure GPD Win back buttons

I have a GPD Win MAX 2 (2024) and would like to be able to use the back buttons in-game as well as to open the HHD / Steam overlay. However, when I change the configuration away from the default I can no longer open the UI panels with the back buttons.

How can I configure the back buttons so that e.g. a normal tap is A, but a long press opens the HHD panel?