The general point of Relativity is that space is simply a measure of what occurs. The three dimensions.

I'm making the point that there has to be some zero frame for it to work.

The reason what time and space dilate in a moving frame is the motion of the frame and the motion within it can't exceed C. So once you add up all the relational aspects, there has to be that underlaying "still" state.

Also there would have to be some definite physical bound to make it finite, without which space is infinite. Goes on forever.

So my sense is these two qualities, infinity and that zero state. Absolute zero in thermal terms.

Otherwise what is the speed of light in relation too, if not this underlaying "equilibrium?"

Empty space is inherently still.

Unbounded space is inherently infinite.