I don't have any experience with split kiteboards but strictly regarding the size question, it mostly depends on the average wind speed you intend to ride in.
If you end up using mostly your 12m and 15m kites which probably means
sub 18 knot winds, then just get a bigger board which will really help you enjoy the lighter wind days but also make it easier to go upwind.
Smaller boards are better for stronger winds, choppier waters, jumping (big airs & rotations especially), but they require more power and more skills.
I weigh about the same as you and will only ride boards
up to 136cm, anything bigger does not make much sense for me as I very rarely ride tips in less than 18 knots.
We have a kite and board size calculator on our website that factors in your body weight, wind range and skill level:
www.kitebud.com.au/how-to-choose-a-kite-size/I hope this helps.
Christian