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 \overrides 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 \overrides.
If you want to keep the little beginning of staff, keep at least one #'transparent = ##t.