I have super struggled with this model and rigging for a month without weekend. I made this rigging object combining copy rotation + transformation. Because I thought some hinges should go faster than others to keep them stick together.
Look at it in Front View (Ortho). Note that with the rig you have it is designed so it will fold into a square, ignoring the end pieces. If you want something else like a pentagon maybe you should start with that shape and work backwards unfolding it. Or just change the shapes of the end pieces to be more like the rest.
It looks like these are tiny math inaccuracies that add up as they go along the edge of the strip, which is why it gets wider towards the end. The 'tail' of your DEL and EL bones are at 5cm and -5cm, and if you make those 3cm and -3cm, the center stays together a lot better. You may need to tweak the size of other bones to get it exactly perfect.