If you've tried the maximum dosage of anti-inflammatory mesalamine in both oral and rectal routes (4,800 milligrams oral like lialda and another 4,000 milligrams via enema like rowasa), then I would consider Immunomodulators like Imura/azathioprine/6MP.
What concerns you about
I ask as I've been on 6MP for 4 years now and have yet to experience a side effect from it. Everyone is a little bit nervous when considering a new class of UC medications, myself included. What helped me was comparing the risks versus the benefits. The odds of 6MP/Imuran improving your UC symptoms is around 70%. The side effects are rare on it, less than 1% of UC patients get serious side effects from it.
If you want to take a deeper dive into what side effects are possible and the odds of getting them, then I highly recommend the following from the Crohn's and Colitis Foundation of America: