I think it is self-evident that matter (“things”, though as a relationalist I don’t think you can talk about particles as self-subsisting objects) and space depend on one another, or to put it another way, they are the one system of relations. Leibniz was theorizing in the seventeenth century, so his formulation of relationism should no be conflated with Rovelli’s relational interpretation of QM. Your problematizing of relationalism seems based on a simplistic ontological divide between spacetime and matter which smells mustily seventeenth century. QFT sees everything as one unified field. The idea of “stuff” is a parochial illusion of apes who evolved to use tools. In stating that it is hard to make sense of what it means to say that everything is relations and there are no things, you just restate western philosophical bias. *You* find this position hard to make sense of because, as a philosopher well and truly steeped in western materialism, you are conditioned to demand some absolute, intrinsically existing thing to ground your ontology. Otherwise you feel like your mind is snapping on air. But as Rovelli points out, this way of thinking has long been completely intelligible to Buddhists. Anyway nobody ever has been able to make sense of what “stuff” is other than how it interacts with other stuff. The idea that there needs to be a Ding an Sich was always just a refusal to let go of the idea of some substance needing to reside at the base of the reductionist hierarchy.