Your growth in the game doesn't stop when you hit an exp cap. Not even close. Hitting max level by itself gives you the incentive to gear up, and to do the content associated with providing you the means to do so. If you rushed your way to 70, you also very likely missed half of the game's mechanics in battle, and when the challenging stuff comes around that reuses them, you might be at a bit of a disadvantage compared to other players who are more readily practiced.

Put quite simply, there is experience in the game that isn't measured with accumulated points or levels. In your position and my mindset, what I would do is actually run every single instance in the game that I have not cleared. I can get to max level easily without touching 70-80% of the game's content, especially after getting there the first time.

If you wanna git gud, then you must always strive to surpass your own limits. If there is an instance that kicks your ass, then you get back on the saddle and head right back in. And as someone striving to get better, you look for that instance that kicks your ass. That is your hurdle to jump.