a little down thrust, as you did, might be all it needs. at this point,.... it's a "try it and see" process,...know the different ways to affect that tendency, is just as important as knowing how to eliminate it.. as said a flat bottom wing will climb when giving some power. you might try just a bit of negative incidence on the main wing, or a tiny bit of positive incidence on the horizontal stab and live with the tendency to climb on throttle. although you shoot for level flight on throttle,...it's not absolutely necessary,.... as long as you know it's there.