I wonder whether brown rice could be contributing to your problems. Brown rice is high in insoluble fibre, and that’s something you need to minimise in your diet, especially when your j-pouch is new. Also, with a new pouch, even potato skin could be too hard on your digestion at this early stage.
Have you tried eating a low-residue diet? If this helps, once you are feeling better, you can gradually introduce a wider range of foods.
Along with the low-residue diet, you could try having smooth Metamucil just before eating. This creates a kind of gel that could help your bowel movements when you are sitting on the toilet. (Please be very cautious trying it.)
I’m sorry you are having such a tough time. Your surgeon or continence nurse should be able to order testing to see why you are having so much trouble with BMs.