Einstein wrote in 1912 :
The principle of the constancy of the speed of light can be kept only when one restricts oneself to space-time regions of constant gravitational potential.
Einstein "did not need the ether" mathematically, so he stopped thinking to make it more complex.
See: The CHAMP-GPS outliers.
See CHAMPs-GPS outliers in:
Experiments to determine the mass related Lightspeed extinction volume
around the Earth and around spinning objects in the Lab.
an example: Comparison of the absolute kinematic orbit solution w.r.t. RSO.
Tae Suk Bae, 2003, Ohio State university.
Reconciliation of GR and QM by a New Gravity-Black Hole and Lightspeed Model Called Quantum- FFF Theory.
The Semi Relativistic Higgs Field Aether with mass related lightspeed adaptation.
See some posters at:
The O‘Connell Effect in Eclipsing Binaries Explained by Mass Related Light Speed Extinction Distances (Lasof) of Stars and Even Planets.