I've been breaking my head open lately over special relativity and its conception of spacetime's dynamical as well as kinematical features. One thing that has stuck in my head is that of whether the concept of relative/absolute simultaneity in a relationist spacetime is different from a substantivalist spacetime? What I mean by relationism and substantivalism is the anti-realist vs. realist distinction between whether spacetime is, generally as i've understood it, reducible or irreducible (has to be taken as a real primitive thing).
Given the assumption that we know spacetime is a real thing, then the concept of relative simultaneity in special relativity or absolute simultaneity in Lorentzian relativity takes on a similar philosophical flavor to presentations of the theory i've seen in other places. What if, however, we treat relativistic spacetime in a relationist guise? Do the concepts of absolute/relative simultaneity change and how? Further, in what ways do these two presentations of simultaneity in different ontological frameworks differ or remain the same?