Urinary tract disease can be very frustrating so I understand what you are going through. There are many causes for cystitis with crystals or stones being just one of them. The crystals your dog has been diagnosed with may be occurring due to the infection or there may be a larger stone in her bladder which has not yet been diagnosed. These are usually apparent on an X-ray. If Opal is prone to crystals developing then she may need to be placed on a prescription diet to alter the pH or saturation of her urine and help dissolve the crystals. I would recommend revisiting the underlying cause of her recurrent infections with the vet and enquiring about a long term management plan.
Best of luck!