Lightest possible USB solar charger with 5V 2A to 3A output?
You can parallel two Solar panels of the same voltage, although the battery pack accepts more power on the USB-C port at 18W=9Vx2A  than the 10W=5Vx2A port.
So I would be looking for a 9V Solar panel in the 25W range which would have an open circuit voltage of 12V (Voc).  Then you need a UBC-C cable and wiring diagram.
However I don't find anything in a quick survey, so I suggest you write to the OEM source of the Solar panels and battery pack.
I did find a wall charger though.