I don't have much advise but understand how difficult this is. My oldest went through massive constipation issues from age 1-3. We had to use miralax and the occasional fleet suppository to get him to go and it was agony. Total agony for all of us.

I know it was frustrating in our case as DS did eat plenty of fiber and got fluids. He just got into a pain/witholding cycle that lead to everything getting so stretched out that he could hold progressively longer and longer. He would get the occasional leakage around the backup (like encoperesis) and he'd end up with "skiddies" as he called them (but these were pretty bad).

Dietary changes really didn't make much of a difference in our case. People would always suggest pear juice, prune juice, more fiber, less fiber, etc. but it didn't matter for us. We did miralax daily and then would use a fleet suppository if we hit 5-7 days without a poop. I don't know exactly how it got better, but it did in time.

We eventually had luck weaning him off of Miralax. I think he stayed on it a year, and then we had another year where he'd be fine most of the time but if he started witholding, we had to make sure it didn't go on for long.

Can you ask your GI about something like xylitol in small amounts? We use erythritol for baking as a sugar sub (which is similar to xylitol in that it is a sugar alcohol.) Erythritol doesn't cause GI upset for most people but xylitol can cause loose stools for some people. I wouldn't do it without asking but maybe you could look into that? There are gum and mints if you wanted to use small doses at first. I'd consult with your GI doc about that first though.

We haven't dealt with this as long as you have but I know it was such a tough thing for all of us. It would have been even more difficult had it persisted into the school years. I hope you can find a solution.