But it is unbalanced...
You should not be able to stack up culture after culture without limit for exactly the same reason as the other wonders. Cultures give you benefits just like other wonders do, some of them better than almost any other wonder.
By the way, if you want to make the game harder, Logik, you should also turn off infinite xp. Having a unit's XP trimmed back when upgraded slows down their promotion rate and reduces the number of super-promoted "I can walk into any enemy city without effort and without siege reducing the defenses" units you have. For units that have over the XP limit (25) it gives you the choice of trying to continue on with the existing unit after the upgrade is available in order to get promotions quicker, or upgrading to get a stronger unit but requiring extra XP to get to the next promotion. Unless, of course, you have a great general attached since they let that unit keep all of the XP it has.
By that same "harder" goal, you should turn on tech diffusion. It helps keep the AIs from falling as far behind in tech as they often do while not really helping the ones that are keeping up (or pulling ahead).
Also, turning city limits from civics back on will probably slow you down a bit as well. They are not hard limits, they just give some unhappiness in all cities if you go over them.
The AI is not so good with surround and destroy either, so turning that off will remove an advantage you have over the AI.
In essence, it is a bit like you specifically pick your game options to make the game easier for you then ask how to make it harder. Stop picking the ones that make it easier and it will be harder.