Similar problems here. What is your baby eating? Sometimes that can make it worse.
Our DD has had to have Miralax+ about 4 oz of juice pretty much her whole life just to keep her moving. I tried just about everything else- different juices (including prune), Probiotics (which for some reason made it worse), different brand formulas once I stopped BF’ing. Nothing else worked for us, and unfortunately she’s also intolerant of milk so the soy formula makes it even worse. Sigh. Now that she’s starting solids, we’ve at least been able to cut the dose back.
One thng our GI specialist told us is that when babies push down to poop, they have some of the worst reflux episodes, so you may see her spit up when she’s trying to go. Unfortunately for us, this exacerbated the problem because then she started avoiding poop on top of the constipation. These poor little kiddos.