Any tips on how to help male cat diagnosed with cystitis?

I adopted my cat at 4 months about a year ago. When we first got him, he was a sickly, malnourished little guy suffering from a lice infestation (he had to get shaved down after two treatments failed) and got some kind of respiratory infection while at the pound. We did everything we could for him and now he’s a plump, happy dude, but he’s suffered from recurring UTIs 5 times this year. Our vet thinks that it’s stress induced cystitis and pretty much said that we won’t know what’s causing it until it happens often enough to factor certain things out. His bladder and urethra get inflamed, and he has medium/high WBC count every time we take him in. When it seems to happen, he strains to pee and paces between litter boxes trying each one over and over. His bowels are usually unaffected though, however this last time he pooped and then strained in a poop stance over and over instead of a pee stance (sorry if that’s tmi idk). He hasn’t had a blockage yet but I’m not overly optimistic that it won’t happen eventually.

He’s on Hills Urinary/Stress diet (wet and dry, but I’m going to switch him to mainly wet w dry to graze on between meals), drinks bottled filtered water from two fountains separate from his food area, has 3 large pellet litter boxes, we have 1-2 feliway plug ins going at all times (950 sqft apartment), and I work from home so he’s only ever home alone when I go out for a few hours every once in a while. We play with him daily, he has a copious amount of toys, cardboard to scratch on, two cat trees, and hiding spots. We do have another 6 y.o. cat that he loves to play with, he’s been attached to her since the moment they locked eyes lol.

Does anyone have any tips on how to further reduce stress or any prevention tools? I know cystitis is a broad diagnosis and it could be something specific, but I’d love to know other peoples experiences before subjecting him to more stress via lab tests. Even recommended tests to ask about during his next vet visit would be appreciated. Thanks!