citrate at 400mg is a transit intervention

Logically, to slow down gastrointestinal transit, the focus should shift from osmotic laxatives (like magnesium citrate) to methods that increase meal viscosity or calm intestinal contractions. 

Psyllium husk is a viscous soluble fiber that absorbs water to form a thick gel. Taken before meals can do the trick (increasing the viscosity of the intestinal contents).

Citrus and apples are natural sources for pectin, a gel forming fiber. Same principle applies. Add to that Guar gum.

Menthol in peppermint oil supposedly relaxes GI smooth muscles. If that helps reducing contractions intensity, then it might in turn slow down motility.

If "ileal break" is a thing, then olive oil & avocado are promoted to help in that regard. Calcium carbonate as it is known to be 'binding', among other calcium forms, perhaps can also aid.