Sorry to hear about your boy kitties. My boy has been on Royal Canin SO for many years and he has had no further issues. It is expensive, but to me it’s worth it - there are always things I can cut back on if needed. I also have two Cat-IT kitty fountains that he and his sister love to drink out of so they get plenty of good water.
Just went thru this with my cat. Since switching to royal canin SO no urinary problems. But he now struggles with constipation