Toddler constipation help?

Li

Any recommendations for a toddler with very bad on and off constipation. And before suggesting prunes, pears, apples, juice, or fiber we do it all. We limit dairy intake and do daily mirilax. She cycles through a couple days of bad constipation then the fiber and miralax hit and she will go several times a day! Any suggestions to help with regularity. Probiotics you’ve tried etc.? We have a doc app next week but looking for more ideas!