To me the important consideration is that a derivation of any wave
equation yields the propagation velocity relative to the medium (the tacit
assumption is that the chosen coordinate system is at rest in the medium).
In manipulating Maxwell's equations into a wave equation in vaccuo - no
particular medium need be chosen; only a (any ) inertial coordinate
system. Here is where an aether medium may be introduced ad hoc - but if
your view is that the equations are valid in any inertial frame (including
the velocity "v" in the Lorentz force) yes, it follows that the E/M
propagation speed is an invariant.