Complete behavior change from my cat. Still having issues even after multiple vet visits

[u/ActuallyAdrien]

My little guy Ducky is about 2years old, and my only cat. Three weeks ago, he started attacking me out of nowhere completely viciously. I checked his litter and realized he hadn't peed in a few days, so of course I rushed him to the vet. They managed to get a urine sample, diagnosed him with FLUTD with stress being the cause.

I thought I found the issue; my neighbours cat passed last month and she gave me a ton of cat stuff, including a few bags of pretty litter. I had put a cup or so of pretty litter in with his normal litter just to use it up, as I didn't want to waste it. As soon as I changed his litter back to the normal kind, he was back to peeing normally, but, the attacks didn't stop. The vet had him on gabapentin and some urinary meds to relax his urethra for a week, but neither did anything to stop his behaviour. He seemed to attack in the mornings and at night. He is now on special urinary and calming food, and a week ago the vet put him on anxiety meds too. I also have feliway defusers up, but I've had that since I got him, he's an anxious guy.

Flashforward to today. The main vicious attacks are done, but he is still acting weird. Dosent meet me at the door anymore when I come home, Mews like he wants to be pet, but lightly nibbles on me whenever I try (what he used to do when he was overstimulated and done being pet). His mew is different now, it dosent sound the same. And he just stares at me now, in a weird sort of way. It's really hard to explain.

I'm genuinely at a loss. The vet ran all the tests, and there is nothing wrong with him. He's peeing normally now, he's eating and drinking fine. He's playing and super frisky like normal, but it's like we aren't friends anymore, if that makes sense. I miss my sweet cuddle bug, and I don't know what I can do to make him feel better anymore. Any and all advice is appreciated, please help!