Early Bedtime/Morning Wake-Ups

Hi, my LO just turned 8 months on Aug. 1 (born 1 month early) and we struggle with him going down for the night without having to go in multiple times to finally get him to sleep. He’ll then sleep great but will tend to wake up bt 3:00 - 6:00 am. Sometimes it’s a quick visit to get him back to sleep and sometimes it requires rocking, snuggling, even a quick nurse. We follow the wake windows from Taking Cara Babies (he’s up to 3-3.5 hours awake before bedtime) but just can’t seem to break the wake cycle consistently. He absolutely CAN sleep through the night, but those nights are not a constant. Any ideas?