You really do need a second opinion, IMO. Make sure it’s with a *board-certified colorectal surgeon* with plenty of J-pouch experience. The advice you’re getting sounds like what a general surgeon might suggest.
The rectum does have some reservoir capacity, but there’s a reason the J-pouch is much more common than the IRA. If the initial problem was UC then the IRA is simply a mistake, because of the downstream cancer risk. The J-pouch is a more technically challenging procedure, though, so the surgeon should be chosen carefully.