Builder should have used plasterboard (dot-n-dab or on batons)
including the arch with metalwork.

Then it comes down to being able to do a decent skim coat.
Skim on plasterboard is not hard - even a crap builder can do it and
just sand the living daylights out of it (just more mess & time, to
his cost as the substrate is perfectly flat).

It may simply be the builder has not appreciated how far the wall is
out.