Yup, Azathioprine works like Imuran. I think you took Pentasa as well?
So the next step for you is indeed Remicade! It has a very high success rate and is often regarded as the ferrari of IBD treatments. But it is very expensive, so doctors usually prefer to keep it as a last resort because most insurance companies wouldn't pay for it anyway unless other lines of treatment had been tried.
In a perfect world, every single IBD patient with a flare could be put on Remicade immediately.
Surgery isn't something I would ever want because I have pancolitis, so they'd have to remove everything and I'd be stuck with a colostomy bag for the rest of my life. I'm in my mid twenties, so I'd prefer a life without one (my father had colorectal cancer and has a colostomy).
However you said only your left side was affected. If it comes down to surgery, they could remove the affected part without you needing a colostomy. There would still need to be changes in your diet, but in the end it's all about weighting the pros and cons.
You're a long way from surgery anyway so don't fret about it too much for the time being.