Why stellar aberration does not depend on the relative velocity:
Earth - star, but only on the speed of the Earth - an observer?
I know that we measure only the cyclic changes of aberrations
(on an annual basis), but that does not change anything here.
But we can not use relative speed here, because then we have a very diverse aberrations of the stars, because of the fairly wide range of speeds that are at stake here.
For example, the star S2, seen in the center of the Milky Way:
v = 5000 km/s, so the angle of aberration in this case, more than 3000'' - nearly 1 degree!
It is incredibly gigantic value - the whole orbit of the star is less than 0.1 '.
I suspect that the principle of relativity is not properly recognized.
Probably here the point is, that the description of the process
in question does not depend on the adopted reference system,
but not the process itself - a phenomenon.