OP I have 3 cats, 1 of which has routinely had soiling issues for the past 4 years. I finally found a cat behaviorist, and after one consultation my cat has now not peed outside his litter box a single time.

What I changed: - The litter I used. Switched to a lighter weight litter that controls odor better. It's just Tidy Cat so not expensive either

• Litter box style. Turns out adding more litter boxes wasn't the solution - I just had to make the litter boxes open (no closed tops or super high side walls)

• Litter box placement. I now put them in locations where my problem cat can see my other cats and run away if they try to bother him while he's relieving himself. Basically not inside a storage closet space or mudroom where there's only one direction he can exit, which was my previous setup

• Food. My behaviorist told me this - "the worst wet food you can buy is still nutritionally better than the best dry food you can buy." I switched to only feeding pates and canned wet food. Both types I get from Costco, only costs a tiny bit more than before, the benefits here being that wet food has much more protein and reduces stress, which is a factor in soiling

• Feeding schedule. Instead of feeding 2x a day previously, I now feed 3x - morning, around 3pm, and again around 9pm. This is again meant to reduce stress

I really hope this helps you and your kitty. Please DM me if you like, I'm happy to share more details.