Do I need to prepare any other syllabus except that of CBSE for IIT or need to have only deep conceptual knowledge about the subjects?

Frankly speaking ..CBSE is bullshit. If you seriously want to clear IIT JEE , you need tons more than just CBSE level knowledge. It would be better if you join a coaching institute.

Well base is CBSE, but surely things will come up you've never even seen in yoir books. By deep conceptual knowledge, it's not about just knowing your theory and being able to solve sums, but knowing your theory and being able to solve every sum that's under that topic and even sums which is a comb