Old Cat (6yo) and UTI Cat (3yo) Diet and Lifestyle Advice

So I recently had my (3yo) cat diagnosed with UTI right after he was neutered. We're not 100% sure if the UTI was a 1 time thing but the vet convinced me to change diet to UTI friendly, saying once crystals start to appear they tend to come back over and over again if no diet change took place. So then I asked about the other (old) cat and he actually recommended that it too should switch as crystals are common with old age. Could anyone with experience or medical knowledge confirm this fully or partially?

Further, I'm currently using Royal Canin Urinary as a preventative measure and to get my cats used to it then I will start a 2 month course with Royal Canin S/O as a therapeutic measure to help remove any build up (if any) then go back to the preventative Urinary food and repeat the 2 month course again later in the same year. It was explained to me this way, could someone please confirm or help me understand this better. Also, is getting my old cat into this type of diet okay or even preferable?

Lastly, I do give them 1 wet meal per day. Its a felix fancy feast as I know it's quite palatable and my cats love it so I take advantage of that with getting supplements into their diet. I use either a GimCat or KitCat UTI pastes and PET360 OMEGA 3+6 solution. I recently found HomeoPet UTI Plus drops and plan to start using it after I'm done with the pastes as they come in a form easier to consume, is this product better than the paste or the same but in different form?

My cats have access to clean running water fountain, Pet Gravity Water Dispenser, and water bowl of which are cleaned regularly and never out of water.

Any additional adjustments or answers to my questions will be highly appreciated. Thanks!