I have opened up the case and took a few photos. It's a fairly simple board with indeed an NE555 timer.
It is however all SMD components, I'm not entirely sure if I can add a cap to change the timing, it seems you know more about it than I do. Would you have a link to the Stackexchange by any chance? I see this circuit in the image is used for 12v, do you know if it can also handle 24v?
Here's an image of the PCB in my dimmer:
