I was diagnosed with UC in December, and my doctor immediately wanted to start me on mesalazine, but after reading about side effects I didn't really want to go down that route.
I'm not a hippy whole earth kind of person AT ALL but I wanted to at least try a couple of things before jumping onto a medicine that makes you gain weight and your hair fall out. So after trying a few different things, I bought a bottle of probiotic that has bacillus coagulans (found it on Amazon). This is a spore-forming soil bacteria, and the literature on it is definitely spotty, at best. All I can say is that it improved my symptoms within 24 hours, and by day 3 I was pretty much symptom free. Since then I've read a few more sources suggesting that this probiotic can have a positive effect on your gut biome, basically outcompeting some of the harmful microorganisms that get in there.
Of course, every single person is different, and I certainly don't think this will be a magic bullet for everyone. But it sure as hell worked for me, and I would recommend giving it a shot. Apart from some occasional minor discomfort when I eat a lot of sugar, I haven't had any serious symptoms for the past 6 months.
Another thing that helped me a bit, before I tried the probiotic, was magnesium supplements. I know for sure that when my symptoms were the worst, I wasn't getting enough vitamin D. First of all, I basically got zero sunlight exposure over the winter, which is the main way your body produces vitamin D. Also, it turns out that your skin requires magnesium to produce vitamin D from sunlight exposure. Apparently lots of Americans are now magnesium deficient, partly due to our crappy diets, and partly due to a decrease in magnesium in grains and vegetables from increased crop yields. Anyway, there is plenty of literature out there showing that vitamin D is important for regulating immune response, so it would probably be worth checking out to see if you're deficient in vitamin D, magnesium, or both.
Good luck. UC sucks, and I really hope you find something that works for your case.