Well, a pressure reducing valve isn't just a flow restrictor.
You can get thermostatic shows designed for the differential
and varying pressures from a multipoint/combi,

I`m not sure what the unit was exactly, a white plastic lump with what
appeared to be a diaphragm and an adjusting screw underneath. I don`t
even know if i`d need something like that in place to make it easier
to control shower temperature (the cold has a much greater flow rate,
so the ability to reduce flow might make it easier to control) - any
loss of cold water flow to the bath itself wouldn`t be an issue as far
as i`m concerned.

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