If that was the case i would have four 15+ cities by turn 100 every time i play, each one producing a wonder every 10 turns.
That's nonsense. There's a big gap between "happiness is hard to get" and "I can build 15+ cities by turn 100, each one producing a wonder every 10 turns".
Happiness is too easy to get with mercantile city-states and religion. I really don't understand why you don't see this.
Anyway, I don't have any ideas to fix Gandhi. I've always thought that his UA was one of the more creative ones, if not exactly one of the more enjoyable ones for me to play. But as long as going tall is so easy (because food caps way before happiness if you're not bad or incredibly unlucky), Gandhi's UA is indeed pretty useless.
Nerf religion!