This sounds so so similar to what happened to us, my eldest wasn’t premature but did have an unidentified virus at ten days old which meant a week in hospital and about a million antibiotics.
I am convinced children whose sleep is that disturbed have digestive problems, so I would ignore all the put her down awake/follow x sleep protocol. This is primarily a medical issue, even if there may not be an obvious fix. In our care, she had CMPA and egg allergy: we clocked it before starting weaning and I cut both out of my diet as she was EBF, but she also had reflux and basically nothing we could do totally eased the digestive discomfort. There were specific baby probiotic drops - bio gaia maybe?- that helped a bit.
ultimately what worked for us was: 1) doing whatever made it bearable. For a long time that was me going to bed at 7pm and my husband sitting up till 1am so I could get a block of sleep. 2) switching to having my husband do the night wakenings for a chunk. I went back to work at 10 months-ish, he took 6 weeks parental leave, and after two or three nights of waking and getting angry at the expressed breast milk her night wakenings dropped massively - she’d still wake, he’d pat and shush her,and she’d go back to sleep. I think at that point she was in a toddler mattress and he was in a double mattress on the floor next to her. I was also incredible anxious about SIDS - the safe seven co sleeping guidance from La leche league helped but tbh I didn’t really relax till baby number two.
the biggest change for us was I think where they start to pull up? Around 9 months? Anyway some developmental stage which was the equivalent of the baby doing crunches, and about two weeks later there was a massive improvement. I just think she had constant low level reflux that was disturbing her just enough to wake her and then that massively reduced.
she grew out of all her allergies, though it took time. Her sleep got better from ten months onwards, though she was still a challenging child to put down and tbh from toddlerhood onwards we basically would go to sleep in our own bed then go into her whenever she woke and sleep beside her in a double mattress on the ground.
she is now 7 and I have to wake her up in the morning. It was an incredibly tough time. There isn’t a magic fix you’re just not finding, but it will get better, and you are loving and comforting your child as best you can. You just need to do whatever you can to get yourself through this next bit and ensure you get the rest you need.