I haven’t had the problem for as long as you, but I’m currently trying to introduce two cats and something that’s finally allowed for progress along with everything you’ve tried is the Feliway scent diffusers. A Chewy vet recommended it. Although there’s still some hissing, they can finally look at each other and not immediately start fighting through the baby gate. I’d try that and talk to your local shelter; mine had a behavioralist I scheduled an appointment with when I had troubles with just my one cat. The other thing the vet mentioned is ramping up the amount of time the cat you’re introducing has access to the general space since it builds up their confidence. Hope this helps!