Desperate, gentle training help

Hi, my baby is 5.5 months old. Up until about 3.5 months he slept pretty well around 5-6 hour stretches at least. Since then, things have gotten to what I think is awful (relative). He wakes typically every 45-90 minutes every single night.

We tried Ferber around 4 months and ended up stopping on night three because he would cry so much he’d throw up after an hour. My son was born with a condition that required surgery to be fixed and spent 5 weeks in the NICU and it’s hard for us to figure out what method would be best for him - he also still suffers from pretty bad reflux which we think may contribute to our sleep problems but unsure - he is on medication for this.

I’m looking for advice for gentle sleep training methods that worked for you around 6 months? For my mental health I need to get more than an hour of consecutive sleep at a time, I also worry this is affecting his development waking so often and having poor sleep.

We typically try to get 3 naps a day in around 30min - 1 hour is how long he’ll nap. I have been sticking to a strict bedtime routine of bath, bottle, book, the bed. The issue is he really still wants to be held before being put down and I think that may be contributing? Honestly just desperate for any advice or methods that worked for you on maybe a more high needs baby? Thank you!!