Curvation of spacetime

[DParlevliet]

But after (meaninglessly) converting time to a distance you then need another time axis if you want to define speed. So this is all nonsense.

No, because the fourth dimention is still t, only multiplied by a constant which makes the reading along this axis the same distance as the other 3D axis. In this model (which is often used) it becomes a 4D-space, which makes calculations easy an understandable. But it is a mathematical model, it does not imply (nor exclude) that time is really a distance. But time itself is also unknown.