If your UC is going to spread in extent or severity then it will within 5 years since your your diagnosis. It's happened to me and others.
You are hardly refractory to all treatments if all you've ever tried is the 1st class of medications, anti-inflammatory mesalamine. There's still the 2nd and 3rd classes of medications to try.
The 2nd class of UC medications is immunomodulators (azathioprine/imuran or 6MP) which are pills you take, much like your mezavant. You require regular blood monitoring while on those drugs, and they can take up to 3-months to work. For those and other reasons, many doctors skip right over them and go to the 3rd class.
The 3rd class of UC medications is biologics, which show a response in 6-weeks or less and do not require regular blood monitoring. I've been on remicade since 2012 and have yet to experience a side effect from it. I have, however, found a remission and remi is the only medication to do this for me. Biologics are nothing like pred. Pred gives everyone side effects. Biologics have rare side effects. What helped me was looking into the odds of risks versus benefits. The Crohn's and Colitis Foundation of America has a good risks versus benefits discussion that looks into the actual side effect odds.
I'd strongly consider biologics given what you describe. We've had posters who've been in remission on remicade for 8-10 years or more (usernames jubjub and red_34).