Let $ABCD$ be a convex quadrilateral with $BC=2$ and $CD=6.$ Suppose that the centroids of $\triangle ABC,\triangle BCD,$ and $\triangle ACD$ form the vertices of an equilateral triangle. What is the maximum possible value of the area of $ABCD$? Express your answer in the form $a + b\sqrt{3}$, where $a$ and $b$ are integers. Please provide the value of a + b.

Let's think step by step and output the final answer within \boxed{}.
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