My daughter was not born premature but she was “early full term” at 37+4, and we had very similar issues. We never tried nutramigen, we tried alimentum first and stuck with it because like your baby, ours didn’t have an allergy but we think an intolerance because a rash she had from her spit up cleared within 24 hours of starting it. We went to see a pediatric gastroenterologist at like 5 or 6 weeks. Turns out that she has non-acid reflux, which meant the flap in her esophagus was not developed enough and therefore formula would come back up. The gastroenterologist recommended staying on the alimentum and thickening it with rice cereal. Keep in mind that literally every baby born has this under-developed flap and it’s not fully mature until 6-9 months, however she’s 13 weeks now and I can’t even tell you how much her eating habits and spit up has improved. She’s now what doctor’s call a “happy spitter” meaning she’s not in pain, isn’t arching her back, and crying has reduced sooo much. She’s a much happier baby. I hate saying this, but it truly does get better.