you should select a board that suits your style of riding and a kite to suit the wind strength. it's your kite that is too big for the wind - not your board

a bigger twin tip is good for beginners because it's more forgiving, easier to drive upwind, and won't bog through the lulls and turns if you mistime the kite handling. as you get better you can handle a smaller twin tip by throwing it around more, edging and releasing and not burying the nose in chop. this opens up more options for tricks.

Yep, your board is too big. You need a 135 x 40.
And something with good (correct) flex. Old tech boards can be a bit too solid. They bounce in the chop and you loose your edging causing you to skip out of the water and loose control.
Try a Shinn Monk or North Select. They'll make a world of difference.
Also lower your kite in higher winds and move your weight a bit more to your front foot . Try and get the entire heel side edge into the water.

Don't overthink it.

Im riding a 165x40 Mako King right now in my 10th season (and 94kgs). The longer length gives great upwind performance and it surfs really well. If you are not enjoying it, try something new.

But if you are losing your edge, maybe you need to change down to a smaller kite ? But a smaller board will also hold an edge better. Just depends what you are trying to achieve.

But if you are still having fun, just keep kiting