Hi there, well the 156 is a very stable platform to learn on and is a good light wind board, especially for non-pumping planing, i.e. when you just wait for the board to plane by itself. When you have a more active pumping style, you can probably also plane in almost as light winds with the 135, as it is quite a bit lighter than the 156, so this offsets the gain of the width that the 156 has.
8-12knots is always very subjective in terms of planing, as this is a threshold where a board will not actually come up onto the plane by itself usually - from 15knots onwards, most bigger boards or sails will plane "automatically", if they are big enough. At 12 knots, you really need a more active pumping style, even with bigger race equipment to get planing. So if you are still at the stage where you are waiting for the board to plane by itself, then the 156 is the better choice to be honest. if you are ready to pump a little and have some technique for it, then the 135 could be big enough. Right now, based on your comments, I would say keep the 156 and invest into a 135 - this may end up being your light wind board in future and who knows, afterwards you may be riding boards closer to the 100L mark once you improve more/sail in stronger winds?
ah, if you do buy a different bigger board for light winds, then I´d suggest something lighter, more like a Gecko LTD 144, or a Falcon Lightwind, which work really well in the light winds you describe - but given your current skill set you describe, better to get the basics right first, as these boards are of course more fragile than the Gecko.