I second beatUC's opinion. I had pancolitis back in 2013 when I was on Uceris and started my Imuran. I worked my way up to 150 mg within 3 months and I was able to get off the steroids and stay in remission.
I would also maybe consider to do the 23andme test and have your results evaluated by promethease. I found out that I would have been a higher risk for cancer on Remicade but my system tolerates Azathioprine very well. FYI I have been on Remicade before and did a "back step". Doing fine
Nausea can be treated with ginger or other medications. Ginger is my best friend. The nausea will lessen over time.