+15 knots and a 15 m2 Chrono with a 1200 cm2 wing - this seems utterly wrong in every way, I think you should start here instead
Unless you weigh 700 pounds, then it sounds right
Racers can ride with such a kite in that much wing yes, but on tiny very flat/fast wings.
You got the inputs how to get the trim right in above post yes, but start using a more suitable kitesize instead, before you mess things around too much.
If you are average weight 75-80 kg in + 15 knots, a 7 m2 LEI is more than sufficient, in fact well powered, on a normal freeride wing.
Thus on a 1200 cm2 you could easily ride with a 5 m2 in this wind - I do this on mine sometimes.
Not saying you should go that small if not into waves, but using a 15 m2 racekite in this wind with a "SURF" sized wing, will make everything difficult.
I assume you have got this size of wing to be able to ride small kites and in low wind, and/or maybe surf - so use the right kites for this for starters, and see how the trim is now, and take it from there
Regarding trimming the stab - I would be very careful with that, as it does not only affect the trim when going straight - it affects hugely how the hydrofoil will carve.
Peter