We can talk about how a region moves in relation to other regions. I’m still not following your objection based on relativity: We have one frame where I’m on

We can talk about how a region moves in relation to other regions. I’m still not following your objection based on relativity: We have one frame where I’m on Earth; we have another where I’m in a rocket traveling away at a fixed velocity. In one case we see the rocket traveling away; in the other, we see Earth traveling away. But neither one tells us that the spacetime itself is moving — they’re both just local coordinates. In the case we see the spacetime itself move, eg with a gravitational wave, both the frame from the rocket and the earth agree that the manifold has been changed in a way that left a wake — moving relative to its local, original shape. As a side note, “inertial reference frames” are a useful fiction: we’re always being accelerated, eg, by the galactic center, fellow galaxies, dark matter filaments, etc. Conclusions based on such a fiction may be practical, but cannot be used to derive conclusions about physical reality — as they don’t exist.