What I want is huge volumes of water at low pressure - we have one of
those "soup plate" shower heads about a foot across.

Yup.

Ah those. I can see that they might be adequetely serviced by a gravity
system, as they do rely on lots of water at low pressure. However, a
standard shower head uses a different system. The multifunction heads always
seem to have a "soft" option, recommended for children (and presumably those
that don't like pressure), that seems to introduce an air break for low
pressure operation, but I find it annoying and never use that mode.

The main problems with high pressure being too much are when you combine
high flow with high pressure, which can get painful. I prefer a middle range
on both. 1-1.5 bar with around 10 lpm seems about right for me. I find less
than 1 bar doesn't have the any pressure washing effect at all.

Christian.