The slowness of light is zero
"The speed of light is zero slowness." Which is to say: the “speed” of light is a misnomer on multiple fronts. It’s not a fixed speed, and we should know that already - just as much as we know that there’s no such thing as a true vacuum. What we call "the speed of light" is only what it looks like to have an almost-entire conversion of mass into energy, and the resulting near-zero "friction" that is a result of such a conversion. The really important question isn’t "what is the speed of light" but what is the behavior of mass and/or energy through space when that mass and/or energy has no friction relative to the fabric of spacetime.
The answer (I believe) is "zero slowness" - or more accurately "near-zero slowness" - and I’ll argue that the concept of "speed" in itself is not a real thing except that it appears to be real, relatively to objects in space with **extremely** high degrees of slowness, such as ourselves. But even from our frame of reference we’ve managed to debunk the idea that the distance between us and (for instance mars) is a static distance. And even from our frame of reference, we know that our entire solar system is "moving" at a "high velocity" which itself is only to say that we have a relativistic velocity relative to other objects, and relative to other solar systems.
If you accept that the universe itself is expanding, then even defining the "distance" between objects is just shorthand for relativistic distance.
To summarize, the answer to the question of the speed of light is thus: the speed of light is near-zero slowness relative to the medium (space, time, and matter) which it is traversing.
Since speed is defined as "time elapsed, divided by distance", and neither "time elapsed" nor "distance" are real concepts. The question, "what is the speed of light" can necessarily not be answerable, except to speak in relativistic shorthand. And in shorthand: the speed of light is near zero slowness, which as well as we can approximate as objects with so much slowness as to be contagious to the spacetime we occupy (slowness contagion, aka "gravity") - from that very slow frame of reference, we approximate the speed of zero slowness (relative to us) is the constant C.