Explicit Drinfeld moduli schemes and Abhyankar's generalized iteration conjecture

Let $k$ be a field containing $\mathbb{F}_q$. Let $\psi$ be a rank $r$ Drinfeld $\mathbb{F}_q[t]$-module determined by $\psi_t(X) = tX+a_1X^q+\cdots+a_{r-1}X^{q^{r-1}}+X^{q^r}$, where $t,a_1,\ldots,a_{r-1}$ are algebraically independent over $k$. Let $n\in\mathbb{F}_q[T]$ be a monic polynomial. We show that the Galois group of $\psi_n(X)$ over $k(t,a_1,\ldots,a_{r-1})$ is isomorphic to $\mathrm{GL}_r(\mathbb{F}_q[t]/n\mathbb{F}_q[t])$, settling a conjecture of Abhyankar. Along the way we obtain an explicit construction of Drinfeld moduli schemes of level $tn$.