Your baby's weight gain from 2.16 to 3.38 kg is a great sign for a preemie. The best way to judge if your milk alone is enough is by how settled your baby seems after a feed, wet nappies through the day, and steady weight gain, not by how much you can express.
Next Steps
Talk to your baby's paediatrician about tapering formula gradually while tracking weight on the corrected-age growth chart.
Health Tips
Please don't stop formula suddenly on your own. A preterm baby needs close weight monitoring while you make this change.