What if it was perceptually at Mach 2 / distance X yet really was at Mach 20 / distance 10 * X ? (Bullshitting numbers here, but you get the point)
Angular speed across FoV is a function of distance from the object and the only thing that makes us understand that high altitude airplanes are nowhere close to a standstill is that we can infer distance from the intrinsic knowledge of the airplane size.
If all observers are close enough together we'd be unable to uncover the illusion, as that would require a widespread enough configuration.
Additionally, I question the "astronomical" attribute, especially with planes cruising at ~800kph, Mach 2 is at most 2.5 their typical speed. Concorde is hardly astronomical, and although jet fighters are not chasing for top speed anymore some of them are still able to go to such Mach factors. Now, Mach 12, that would be closing in to being astronomical (entering the order of magnitude of an ICBM descent speed).