These are hard decisions, balancing risks, benefits, and costs, each with murky probabilities. If you try to simplify the decision by focusing on one part of this you end up with a simple-but-false conclusion. A good decision requires allowing all of that information in without becoming overwhelmed by it, but that’s easier said than done.
The risk of infertility from the surgery is real, but it’s not enormous. The fact that you are (sort of) in control of it can exaggerate its importance. The risks (including to fertility!) of not having the surgery are just as real, but may seem less like the result of a decision you control, and more like the result of a disease that you don’t control. That perceived lack of control can cause us to underestimate the risks from the thing we don’t do. It’s important to compare the paths as honestly as we can manage, which means risks against risks, and benefits against benefits. For example, the path you describe as “tough it out” could instead become “emergency surgery while pregnant.” I hope not, but UC doesn’t care what either of us hope. It’s particularly important to not simply compare the best possible result of one choice against the worst possible result of the other.
The surgery would reliably consume months of your life, but UC can (unreliably) do that, too. Is this better (less overwhelming) with one baby at home or two?
The risk of colon cancer from UC becomes quite enormous as the years go by, eventually becoming similar to risk of lung cancer from smoking. Colectomy eliminates that risk, by and large. But the real decision you are describing is the risk of delaying surgery a bit, which is more nuanced.
I’ve jumped around a bit here, but I hope some of this is helpful as you consider your choices. You have described a case of UC that just won’t quit, with likely-ineffective management strategies left to you. When I ran out of medical options I moved on to the next best choice, but your mileage may vary. I hope you do very well on whatever path you choose.