Fed up and need help

This is probably going to be a long post but I want to include all the details I can. I took in a male cat in the fall of 2018, who I will call L. He was feral caught and I spent 2 years trying to tame him with great success. He has since gone from a very aggressive cat to a big cuddle bug that loves attention. I already had a dog and 2 other cats before bringing him home, which he never had issues with. In 2020 I lost one of my cats and my friend surrendered one of her outdoor cats to me because she was afraid he'd get killed by coyotes. L took to the new addition immediately and they became bonded. Unfortunately, I live in an area where people love to dump their unwanted animals and I have too big of a heart to leave them so I since have gained 3 more cats, leaving me at a total of 6 cats and 1 dog. L has never had any issues with the other animals and is super friendly with all but 1, who is a loner and avoids the rest of the them. The last addition was a kitten born in 2022 to one of the cats that was dumped two months prior and I brought in suspecting she was pregnant.

From the moment I brought L home he had not had any issues using a litter box and things were fine until the summer of 2023. I ended up having to spend a week in another country and a friend watched my pets for me while I was gone. This friend has been around for the entire time I've had the cats and they know this person well, so I don't believe the issue was with their caregiver or anything they did during my absence. I checked my security cameras multiple times throughout the day while I was gone and all seemed well aside from one of the cats crying and looking for me (I had never been away from him for more than 8 hours since he had been born). My friend never mentioned anything being wrong and everyone seemed fine when I came back. However, shortly after I came back I noticed a new behavior from L of peeing outside of the litter box. He peed on my bed multiple times and on my clothes a few times which resulted in all of the cats being banned from my room altogether. He now pees on anything that resembles cloth if he can get to it (ie paper towels, dryer sheets, etc) and pees on the litter mat outside of the box. The confusing part is he still actually goes in the litter box and uses it too. I've tried everything to curb this behavior with no success. None of the other cats have any issues, he is the only one peeing outside of the box.
I've taken him to the vet multiple times and all they tell me is they can't see anything wrong and give him antibiotics with the hope that it's a UTI and will go away. They saw no obstructions or anything. My pets have a running water fountain that is filled with bottled water and I was told dehydration wasn't an issue with him. After the last vet visit they decided to diagnosis him with FIC since they couldn't find any other explanation, but none of the medication they give him helps. I even had them prescribe him pain meds (gabapentin) thinking he could be arthritic but that did nothing. They said if the medicine isn't helping him then it could be behavioral but I can't figure out what is causing this behavior or how to stop it.
I have tried everything I know to do and nothing is working. I've tried positive reinforcement by giving him treats every time I saw him go to the box but all that did was teach him to fake going to the box to get more treats. I tried swapping the litter thinking maybe he developed a dislike to the one I used but ultimately ended up back with the one he's always used. I've tried multiple different litter boxes as well. He started out with a small plastic one and was upgraded to an extra large plastic one. I then switched them to a covered metal one and also tried taking the cover off. I've tried multiple litter boxes but they all end up using the same one. I even thought maybe he was unhappy with how often I cleaned the boxes so they now have a (super expensive) extra large self cleaning litter box that dumps as soon as they leave the box. I even broke down and tried the pheromone diffusers to exhaust all my options but nothing is stopping him.
We have not moved, there have been no further additions to the household, and there's been no other major changes aside from that one week that I left the country. I can't see this behavior being linked to that and still continuing 3 years after my return. I am at my wits end and am looking into rehoming him if I can't find a cure. Does anyone have any suggestions?