Hi Justine, and hi Irwin! Irwin is such a handsome guy!
What you describe sounds very much like my Luna's history, she started dealing with UTIs when she was 2, and was eventually diagnosed with struvite crystals.
You've already got a lot of information here, but what I'd like to add is that I think you should contact the vet who diagnosed the crystals and ask which kind of crystals Irwin had. That's important for the treatment. With struvite crystals (the ones Luna has), it's often that the pH-value of the bladder is too basic, so you can help the cat by adding an acidic supplement to the food (I have a paste for Luna called Uro-Cyt). With calcium oxylate crystals, it's the other way around, I think, but I'm not sure.
Additionally, Luna is getting homoeopathic remedies like Cantharis or Berberis when she has an episode, and it seems to help her to recover. Many others made some good experiences with Cranberry extract.
Judging from the fact that your vet recommended Hill's c/d, I think Irwin has most likely struvite crystals as c/d is low on magnesium. Low magnesium is always part of a struvite crystal diet.
Be prepared that Irwin might always have episodes where the UTI and the crystals return. These things can easily become chronic. It's the same with my Luna. It's important to watch these cats closely, as the crystals may cause a blockage, which especially may happen in male cats because of their longer urethra. That's why some males get their penis amputated, which really seems to help them.
Anyway, it's a long road to go, and many here are dealing with it. It's often frustrating, but you'll get a lot of advice and moral support here!