The matroid you're considering is the direct sum of the matroids where you take the $k$th wedge power, so it suffices to study those for each $k$.
The question of whether, for an abstract matroid $M$, there is a matroid $\hat{M}$ with the properties that you expect has been studied in the paper "Glueing matroids together: a study of Dilworth truncations and matroid analogues of exterior and symmetric powers" by Mason. I'm not sure if there is a counterexample there (as the paper is hard to access), but one expects the answer to be negative, as there is a counterexample to the analogous question for tensor products in the paper "On products of matroids" by Las Vergnas.
Brakensiek, Dhar, Gao, Gopi and I have shown that if the field has characteristic $0$ and $\rho$ is generic, then the dual of the matroid for $\wedge^2$ is Kalai's hyperconnectivity matroid. This matroid has been studied a fair amount, but in general there is nothing you could call a combinatorial description of it.
Here you really need $\rho$ to be generic, representing the uniform matroid is not enough: there are realizations of the uniform matroid in characteristic $0$ for which $\wedge^2$ is not dual to the hyperconnectivity matroid. If $\rho$ is generic and the field has characteristic $p$, then I don't know if you still get the dual to the hyperconnectivity matroid.