Brief and simplified discernment on the nature of light to complement the following research https://www.technology.org/2021/01/11/there-is-no-way-to-measure-the-speed-of-light-in-a-single-direction,
Speed of light is not constant or assuming light speed is constant is neither realistic nor existential possible for the following main reasons:
1.0 The cosmos is not a vacuum. And even a vacuum is not or never empty.
2.0 There are plenty of observable and currently unobservable matter and spectra-field in the cosmos.
3.0 There are constant dynamic and transformative cosmic interaction from the infinitely finite to the infinitely infinite depending on comes topology, structure, function, and order.
4.0 Light as known or defined does not exist by itself or is casual effects from a interdependent and interconnected irreducible whole and greater existence. Another way of saying is light is part of broader spectra-field. And not only observable but also unobservable spectra-filed. A simple way of saying is light is part of the unified spectra-field.
5.0 And from 4.0, light has many embedded and embodied or subtle characteristics of spectra-field such as energy, frequency, color, phase, wave length, vibration, rhythm, wave function, and so on. Another way of saying is attributes and traits of the greater whole from the ground of being. And ground of nature (as a brief and simplified illustration https://www.epistemologyontologyfoundationinstitute.org/nature-decode/on-being-and-becoming-in-the-known-physical-world-and-beyond). And therefore, are of infinite potentialities of possibilities beyond speed.
And as such, the conditions of the assumption can never be true and existential or realistic. Furthermore beyond these, light or spectra-field exist as part of the cosmos universal process and mechanism based on broader topology, structure, function, and order in the cosmos (such as the essence and nature of "Dependent and Interdependent Origination", "Dependent and Interdependent Arising". "Parts of Sum of Parts of The Greater Whole and The Whole Is Far Greater Than Sum of Its Parts" rather than just simply based on current narrow definition or assumption. Another way of saying is spectra-field (is beyond the definition or feature of light) has broader functions as part of universal process (and mechanism) of transformation, evolution, and optimization or of being and becoming rather than just a static cosmic feature or constant. Otherwise, the cosmos would never exist much less existed a static system.
And there have been many recent evidential and empirical data through applied sciences in recent years from slowing down light or wave function (through diverse mediums-entities) to speeding up light or wave function through diverse freedom and function of entanglement (such as through polarization of light or spectra-field). And addition, complementary empirical data from the human experience through such as OBE and NDE indicated beyond any doubt that light is not a constant as well as limited to current defined speed.
Link as complementary elaboration: The Mathematics of Consciousness - Emerging Science of Consciousness - EPISTEMOLOGY OF LIFE AND ONTOLOGY OF EXISTENCE FOUNDATION AND INSTITUTE (epistemologyontologyfoundationinstitute.org)
Additional scientific and empirical fundamental reasons are space-time *and therefore, the wave function) are irreducible, non-linear, and emergence.
Epistemology of Life and Ontology of Existence Foundation and Institute