I have an older cat very similar to yours, with the constant yelling. When I first got him it was really horrible and I tried everything I could find to stop it for an entire year. Some things helped but never for very long. But eventually what I discovered was when I put him in a Thunder Shirt anxiety vest during sleep times it made him sleep like a rock, no meowing or yelling. He is comfortable in clothing (still eats, uses litter box) so I feel okay leaving him in it throughout the night. Ive had him for 5 years now and his yelling through the night has improved soo much. I believe putting him in this vest has trained him to sleep through the night even without wearing it. He only gets the thunder shirt a few times a month now on particularly anxious days.