Jim Black wrote:

That's why the velocity of galaxies away from us is proportional to
their distance (until relativistic effects kick in, and their relative
velocity compared to us becomes ambiguous).

Relative velocity does not become ambiguous when relativistic effects
kick in. There might be a bit of extra work involved in establishing
precisely when and where the relative velocity happened or how long it
lasted, it is all very definable and not ambiguous at all.

Events can be described in space and time very precisely according to
an agreed upon reference frame, just as we on earth all describe time
on earth according to GMT.