Both the lead sounds you used (accordion and violin) are sustaining sounds that real players vary the volume of over time on held notes. Perhaps you could lower the Key Touch (velocity sensitivity) to Medium if it is not there already, or even to LOW (but you'll probably need to dial down the Filter Cutoff a tad), and use an expression pedal (EV-5/7) to adjust volume during the note?

A lower Key Touch setting will even out your played notes, and the expression pedal can do the rest for you...