To add an extra staff after the start of a piece, you can choose between having brackets or braces when you go over a new line:
=> Use PianoStaff
instead of StaffGroup
or ChoirStaff
in the code.
You may wish to remove the clef and not only the key:
=> See both \override
s in the code.
You may wish to remove also the little part of staff at the beginning of the extra staff:
=> Use #'stencil = ##f
instead of #'transparent = ##t
for both \override
s.
If you want to keep the little beginning of staff, keep at least one #'transparent = ##t
.