Most of us have an opinion of what spacetime is, and a brief summation of my own opinion would be this...

Time is not a standalone phenomenon, Einstein and Minkowski made that abundantly clear to us all. they both stated that spacetime, is a single phenomenon, which should not be mistakenly perceived as two separate phenomena. Unfortunately Minkowski appeared to have contradicted himself, with his postulate of three dimensions of space, and one of time, representing spacetime.

Minkowski spacetime is actually a mathematical coordinate system, which can be used to determine the location of other objects within __this__ instance of space-time, __relative__ to the observer. I don't think Minkowski ever intended anyone to accept it as an explanation of the fundamental nature of spacetime.

I concur with Einsteins' postulate that space-time constitutes a geometry within which spacetime can be curved by the proximity of matter/mass with gravitation being the effect of that curvature. John Archibald Wheeler got it right when he said that... Spacetime tells matter how to move; matter tells spacetime how to curve.

The metric/scale of spacetime is not fixed. Space-time can be expanded or compressed, but the ratio of spacetime (aka the speed of causality) is fixed. this is why the speed of light appears the same to all observers, and the ratio also prevents an effect from preceding its cause. Or, as Minkowski put it, "preserves an independent reality."