You have waded into an old thread and an old post I made in 2018. It's now 2022 and my J pouch has turned 30 years old while the rest of me is now 59. So it's now over half of my life with a J Pouch.
The Fleet enemas tend to be something of an irritant to the pouch and in the last few years I emptied the saline solution and used warm water enemas instead. You are constipated so we have to distinguish between laxative effect and just cleansing the pouch. You may want to use the Fleet saline solution, which is a laxative, as a treatment of the constipation. The caveat is it may cause some mild irritation of the pouch. I am not familiar with "drinkable fleet" but there are myriad drinkable laxatives and I usually am told to drink Mag Citrate, 1 10 ounce bottle. This will result in about 2 hours of explosive watery bowel movements and totally cure your constipation and clean you out. It's got a horrible taste and needs to be chilled and chugged, the "C and C method." Just make sure you get the clear liquid and no dyes.
What concerns me is why you are getting constipated. Are you taking pain meds, Imodium or other bowel slowers? You should eliminate anything that's slowing down the bowels and hydrate, which you should be doing copiously if you take any laxative anyway.
Good luck with your scope.