I’d echo what dlewicki wrote. If you want to add bulk to the stool ( I agree that watery stool is unpleasant), then psyllium fiber (like Metamucil or Konsyl) is often the best way to do that. I use a bowel slower (Lomotil, like Imodium) when I actually want to slow things down (e.g. at bedtime or before getting on a boat).
Straining can do you some real harm, so it’s best to avoid it if at all possible.
The word “diarrhea” means so many different things to different people that I find it just confuses things. As long as I’m not having urgency or excessive frequency I only pay a little attention to stool consistency, but the ideal consistency for most J-pouchers is something like pudding or applesauce.