Ditto the stool test thing, clostridium difficile can sneak up on us and doesn't respond to UC meds at all. Rather c diff requires antibiotics to beat (steroids and immune suppressors make c diff worse.)
What's the fear in combining simponi, azathioprine, and pred simultaneously? You still have a functional immune system even with those meds combined, you're not going to be a bubble-boy/girl on them lol. I did that with remicade, 6mp (remicade is a sister med to simponi, 6mp is a sister med to azathioprine), and pred simultaneously and I was fine. Our immune system needs to be taken down a few pegs at times.
I am not as familiar with Simponi. I would ask if the frequency can be increased between Simponi shots (I believe they have you on monthly shots in maintenance phase, but twice a month should be possible if you ask). I would give Simponi 6-8 weeks for signs of an initial improvement, and a longer series of months for a full remission. I would ask about
having a blood test to check your Simponi levels and whether you have any anti-simponi-antibodies developed. Not sure your country or what labs are near to you, however, here's the lab test I can get in my area for it: https://www.labcorp.com/test-menu/39041/golimumab-and-anti-golimumab-antibody
65% of us respond to a biologic like Simponi, give it a fair shake of 6-8 weeks, and then move on to another (as, unfortunately, nothing works for 100% of us).