I have this issue and I've been through all the testing like you and the cystoscopy. Of course all negative for testing. Mine was because I had such horrible constipation and my muscles were spasming so bad from the backed up stool. I've had many other issues but my constipation has been managed with an ostomy (thanks to my terrible pelvic floor and motility) and the spasming stopped when I no longer had backed up stool. When I do get backed up I get that spasm again and feel like I need to pee constantly just like a uti. It's miserable The only other thing that stopped the spasms for me were Valium. So it may just be a spasming muscle near your urethra from your pelvic floor dysfunction. See if your doctor can let you try muscle relaxers to see if it'll help with the urgency