I’ve had good results with psyllium in powder form (rather than capsules). The wafers worked fine, too, but I don’t want quite so many cookies in my diet.
Because the loop ileostomy is upstream it bypasses the last part of the small intestine. For many folks this makes a considerable difference in nutrition and stool consistency. In addition, the optimal consistency for a loop ileostomy may not be the same as what’s best for your J-pouch. If you have a current need then by all means address it, but the experiments will more or less have to start over after takedown.
Good luck!