This isn't an issue with the dungeon designs themselves, or content being "too easy". It's a problem with the FFXIV core encounter design. Fights in FFXIV are based entirely around pattern recognition. Anyone who is even half decent with recognizing patterns will understand any normal fight after experiencing it even a single time. Once you get it, that's it, you get it. You can just repeat the same thing over and over and be successful. Since the fights never change neither does the way you play them. And since nearly all job rotations are static as well this simply exacerbates the problem. You are literally just doing the exact same thing, at the exact same points in the fight, every single time.

Without some variation with how combat is designed this will always be an issue. It's not a case of making the fights "harder", it's about giving the players something to think about after the first clear.