Hi and welcome to Share. Congrats on your sweet girl graduating from the NICU. I'm sorry to hear she's having such a hard time. First of all... you're not a failure. It's hard to always know what the right thing is to do to help our babies. The fact that you're here seeking support and advice tells me your baby girl is very lucky to have you.
Now... to your question. All that iron is hard on the tummy. I'm not a nurse, but I am the mom to 4 children (my twins were born at 25 weeks). Her pooping schedule doesn't sound unusual at all. If I'm reading this correctly she's pooping every day to day and a half. That's normal. However her crying so intensely doesn't seem right. Is she crying, as she's pooping? If yes, that's concerning. If she's crying for extended periods of time and seems mostly inconsolable she may be experiencing colic or tummy upset from her food. My one preemie had to be switched from NeoSure to Alumentum formula, when she wasn't having breast milk. The neosure was too hard on her tummy and she cried for long periods of time. As soon as we stopped that formula and put her on the Alumentum she was like a new baby.
And in regards to your pediatrician's office not being responsive I would tell them you're not happy with their lack of response, especially given your baby spent time in the NICU. If they still seem to be less than supportive I would explore finding a new practice if you can do that. Their lack of response is not something you should have to deal with.
Good luck and hang in there Momma!