% add an additional section level between part and chapter.
\usepackage[newparttoc]{titlesec}
\usepackage{titletoc}

% \titleformat{\part}[display]{\normalfont\Huge\bfseries}%
% {\partname~\thepart}{1pc}{\Huge\bfseries}

\titleclass{\subpart}{page}[\part]

\newcounter{subpart}[part]
% \numberwithin{chapter}{subpart}
\renewcommand{\thesubpart}{\thepart-\Alph{subpart}}
\newcommand{\subpartname}{Subpart}

\titleformat{\subpart}[display]{\vspace*{6cm}\centering\normalfont\Large\bfseries}%
{\subpartname~\thesubpart~}{1pc}{\LARGE\bfseries}
\titlespacing{\subpart}{0pt}{0pt}{0pt}

\titlecontents{subpart}[0pt]{\addvspace{1pc}\normalfont\bfseries}%
{\thecontentslabel\enspace ---\enspace}%
{\normalfont\bfseries}{\hspace{2em plus 1fill}\contentspage}