Hi there, I had the colectomy and j-pouch surgeries at ages 14 and 15 and gave birth (vaginally) to my son at 28. (He's 15 months old now, so this was last year.) I did not find that pregnancy was affected by my pouch/medical history at all. At most, I think scar tissue may have caused me to carry the baby all in the left side of my abdomen (or maybe he just liked being over there?), which put some excess strain on my back/left shoulder, but obviously this was a pretty minor issue in the grand scheme of things. I was very nervous about the vaginal birth/c-section question as you mentioned above, but I had doctors who believed my best shot at minimizing risks was to try for vaginal and do everything we could to reduce the risk of tearing. I ended up with a 2nd degree tear, so it was confined to my vagina and had no impact on my rectum/pouch. Also, this wasn't part of your question, but I also wanted to note that I did have pouch-related infertility. We ended up going through IVF in order to get pregnant.
Happy to talk more if you like, but all this is to say that I understand your fears. There are a lot of unknowns going into pregnancy/birth, and no one can predict the outcome. It's especially frustrating with our complicated medical histories, when it feels like there's no standard set for how we should proceed. All we can do is research, consult with the people/professionals we trust, and make our best-informed decisions. Wishing you the best of luck!