I thought of another way to explain my thinking. Consider the following... in every direction we look, we observe the past. Perhaps a nanosecond ago, perhaps 8 minutes ago, perhaps billions of years ago. Every single direction in 3d space is the past. Consider walking from A to B. When you get to B, are you in the same place that B was when you started your journey? Of course, B has moved, in time (and therefore space).

So in what direction is our motion? We're moving into the 4th dimension... the future. Everything (with mass) moves into this unseen dimension. Time is expansion.

We don't move into pre-existing spacetime. Two objects with mass (let's say the Earth and Moon) are both moving into new spactime, and both resist this motion, creating curvature. This curvature keeps both bodies entangled in space, which is why we don't expand away from each other at light speed.

A photon can change it's wavelength to change momentum without changing speed. You'd need to do something similar, but IDK what that means for a non-photon.
Yeah I mean this is excellent. I can't think of another way for something with mass to change momentum other than to accelerate.

You are trying to unify QM and GR in an ex-poker forum.
It's more fun than massaging my ego on flat earth youtube videos.