CASTOR3D linear extended-magnetohydrodynamic calculations examine external kink and peeling-ballooning instabilities in a high-$\beta$ quasi-axisymmetric stellarator equilibrium. Numerical improvements, increased computing capacity, and a high-fidelity GVEC equilibrium permit substantially higher dominant toroidal harmonics $n^$ than earlier studies. Low-$n^$ modes strongly couple the positive- and negative-Fourier branches and can form distinct slow- and fast-growing locked solutions; this coupling weakens as higher-$n^$ modes localize, making a reduced eigenproblem adequate. Ideal growth rates rise with $n^$, whereas parallel viscosity lowers growth and flattens this trend without unlocking modes. Gyroviscosity, ion-diamagnetic drift,
- Extended CASTOR3D stability calculations to high-$n^*$ modes in a three-dimensional quasi-axisymmetric equilibrium.
- Characterized Fourier-branch coupling, locking, rotation, and localization across the mode spectrum.
- Quantified how parallel viscosity changes growth rates and dominant mode types without inducing oscillation.
- Resolved the combined influence of gyroviscosity, diamagnetic drift,
$E\times B$ flow, and external flow. - Showed that flow magnitude and direction can switch the equilibrium's most unstable mode family.