How can I keep sectioning but disable its display

Question

I'm quite happy with titlesec and titleps for keeping track of sections and creating footers that are conditional on whether a chapter start or is continued, but there was a case where it wasn't straightforward to obtain a custom format. So, what I'm looking to do is decouple sectioning from its format. As a workaround, for now, I just want to disable any typesetting of \chapter{...}. In the example below, this means getting rid only of "Chapter 1\nl Fonts", such that "SOME CUST CHAP. FORMAT" comes at the top of the same page.

\documentclass{report}
\usepackage{fontawesome}
\usepackage{lastpage}
\usepackage{tabularx}
\usepackage{titlesec}
\usepackage{titleps}
\usepackage{xparse}

\newcolumntype{R}{>{\raggedleft\arraybackslash}X}
\newcolumntype{C}{>{\centering\arraybackslash}X}

\NewDocumentCommand{\thedoc}
{}% Is there not a command for `token list` in xparse?
{THIS DOC}

\NewDocumentCommand{\myheaderii}
{+mm}
{%
  \begin{tabularx}{\textwidth}{@{}XR@{}}#1&#2\end{tabularx}
}  
\NewDocumentCommand{\myheaderiii}
{+mmm}
{%
  \begin{tabularx}{\textwidth}{@{}X@{}C@{}R@{}}#1&#2&#3\end{tabularx}
}  
\NewDocumentCommand{\chapterMark}{}{UNDEFINED!}
\NewDocumentCommand{\setchapterMark}{m}
{%
  \RenewDocumentCommand{\chapterMark}{}{#1}
}
%\setcounter{chapter}{0}
\NewDocumentCommand{\newChapter}{mmO{}}
{%
  % \stepcounter{chapter}
  \chapter{#1}
  \setchapterMark{#2}

  SOME CUST CHAP. FORMAT%THAT NO EXISTING PACKAGE CAN EASILY MAKE
  \textless#1\textgreater
}
% pagestyle--------------------------------------------------------
\newpagestyle{specialii}
{
  \setfoot{%
    \myheaderii{\thedoc}{\thepage/\pageref{LastPage}}%
  }{}{}
}
\newpagestyle{specialiii}
{
  \setfoot{%
    \myheaderiii{\thedoc}{\chapterMark}{\thepage/\pageref{LastPage}}%
  }{}{}
}

\assignpagestyle{\chapter}{specialii}

\begin{document}
\pagestyle{specialiii}
%\pagestyle{empty}
\begin{titlepage}\vspace*{\fill}\huge\begin{center}\thedoc\end{center}\vspace*{\fill}\end{titlepage}

\newChapter{Fonts}{\faFont}

Let's make that span\newpage another page

\end{document} 

Answer 1

Here is a proposition, certainly to be adjusted, as some questions have to be answered: will the chapter title replacement text be a fixed text for all such layouts or will it be just a new chapter title without the label? What about the tableof contents and the header/footer?

 \documentclass{report}
    \usepackage[utf8]{inputenc}
    \usepackage[T1]{fontenc}
    \usepackage[a6paper, margin=20pt, bottom=34pt, footskip=24pt, showframe]{geometry}
    \usepackage{fontawesome5}
    \usepackage{lastpage}
    \usepackage{lipsum} % Dummy Text
    \usepackage[pagestyles, explicit]{titlesec}
    \usepackage{xparse, etoolbox}
    \usepackage{tabularx}
    \usepackage{hyperref}

    \renewcommand*\familydefault{\sfdefault}

    \NewDocumentCommand{\chapterLabel}{}{Ch. \thechapter}
    \NewDocumentCommand{\chapterMark}{}{U HAVE TO DEFINE IT!}
    \NewDocumentCommand{\setchapterMark}{m}
    {%
      \RenewDocumentCommand{\chapterMark}{}{#1}
    }%

    \newcolumntype{R}{>{\raggedleft\arraybackslash}X}

    \newpagestyle{main}{%
      \setfoot{\thepage/\pageref{LastPage}}{}{\chapterLabel}
    }%

    \newpagestyle{special}{%
      \setfoot{\thepage/\pageref{LastPage}}{}{\chapterMark}
    }%
\newtoggle{nodisplay} 

\titleformat{\chapter}{\thispagestyle{empty}\sffamily\large}{\iftoggle{nodisplay}{}{\rlap{\makebox[\textwidth][r]{Ch. \thechapter}}}}{0em}{\iftoggle{nodisplay}{\vspace*{-8.5ex}}{#1}}[\leavevmode\togglefalse{nodisplay}]
    \titlespacing*{\chapter}{0pt}{-3ex}{4ex}%-5ex

    \pagestyle{main}

    \begin{document}

    \title{Lorem Ipsum}
    \author{Dolor S. Amet}
    \maketitle
    %
    \chapter{Gibberish}
    %
    \lipsum[1-5]{
     \toggletrue{nodisplay}
    \chapter{More Gibberish}}
SOME CUSTOM TITLE \par
    \lipsum[6-12]
    \clearpage

    \chapter{Still More Gibberish}
    \lipsum[6-12]
    \clearpage

    \setchapterMark{\faBook}
    \pagestyle{special}
    \chapter*{Bibliography}
    Let's make that
    \newpage
     ... span a second page

    \end{document} 

Answer 2

Here's my attempt at adapting @Bernard's answer the MWE of this post:

\documentclass{report}
\usepackage{fontawesome}
\usepackage[showframe]{geometry}
\usepackage{lastpage}
\usepackage{tabularx}
\usepackage[explicit]{titlesec}
\usepackage{titleps}
\usepackage{xparse,etoolbox}

\makeatletter\def\@makechapterhead#1{}\makeatother

\newcolumntype{R}{>{\raggedleft\arraybackslash}X}
\newcolumntype{C}{>{\centering\arraybackslash}X}

\NewDocumentCommand{\thedoc}
{}% Is there not a command for `token list` in xparse?
{THIS DOC}

\NewDocumentCommand{\myheaderii}
{+mm}
{%
  \begin{tabularx}{\textwidth}{@{}XR@{}}#1&#2\end{tabularx}
}  
\NewDocumentCommand{\myheaderiii}
{+mmm}
{%
  \begin{tabularx}{\textwidth}{@{}X@{}C@{}R@{}}#1&#2&#3\end{tabularx}
}  
\NewDocumentCommand{\chapterMark}{}{UNDEFINED!}
\NewDocumentCommand{\setchapterMark}{m}
{%
  \RenewDocumentCommand{\chapterMark}{}{#1}
}
%\setcounter{chapter}{0}
\NewDocumentCommand{\newChapter}{mmO{}}
{%
  \setchapterMark{#2}
  \chapter{#1}
}
% pagestyle--------------------------------------------------------
\newpagestyle{specialii}
{
  \setfoot{%
    \myheaderii{\thedoc}{\thepage/\pageref{LastPage}}%
  }{}{}
}
\newpagestyle{specialiii}
{
  \setfoot{%
    \myheaderiii{\thedoc}{\chapterMark}{\thepage/\pageref{LastPage}}%
  }{}{}
}

\newtoggle{nodisplay} 

\titleformat{\chapter}
{
  \thispagestyle{specialii}
  \sffamily\large
}
{
  \iftoggle{nodisplay}
  {}
  {
  }
}
{0em}
{
  \iftoggle{nodisplay}
  {
    %\vspace*{-8.5ex}
  }
  {SOME CUST CHAP. FORMAT%THAT NO EXISTING PACKAGE CAN EASILY MAKE
  \textless#1\textgreater}
}[\leavevmode\togglefalse{nodisplay}]

\titlespacing*{\chapter}{0pt}{-3ex}{0ex}%-5ex

\begin{document}
\pagestyle{specialiii}
\begin{titlepage}\vspace*{\fill}\huge\begin{center}\thedoc\end{center}\vspace*{\fill}\end{titlepage}

\newChapter{Fonts}{\faFont}

Let's make that span\newpage another page

\end{document} 

Answer 3

Here's an answer that is closer to the problem specification as it disables the display of the chapter header, and that only. I gave an example here in which customizing chapter header through titleformat probably not doable. I added the toc to show it is unaffected. A few cosmetic TODO's remain within the code...

\documentclass{report}
\usepackage[french]{babel} % added to problem specification
\usepackage{fontawesome}
\usepackage[showframe]{geometry}
\usepackage{lastpage}
\usepackage{tabularx}
\usepackage{titleps}
\usepackage{xparse}
\usepackage{xpatch}

%https://tex.stackexchange.com/questions/511253/how-can-i-keep-sectioning-but-disable-its-display

%disabling chapter display----------------------------------------
% \makeatletter\def\@makechapterhead#1{}\makeatother
% This one does:
\usepackage{xpatch}
\tracingpatches
\makeatletter % the macro name contains @
\patchcmd{\@chapter}{\if@twocolumn
                      \@topnewpage[\@makechapterhead{#2}]%
                    \else
                      \@makechapterhead{#2}%
                      \@afterheading
                    \fi}{}{}{}
\makeatother

%-----------------------------------------------------------------
\newcolumntype{R}{>{\raggedleft\arraybackslash}X}
\newcolumntype{C}{>{\centering\arraybackslash}X}
\NewDocumentCommand{\thisdoc}
{}% TODO Is there not a command for `token list` in xparse?
{THIS DOC}

\NewDocumentCommand{\myheaderii}
{+mm}
{%
  \begin{tabularx}{\textwidth}{@{}XR@{}}#1&#2\end{tabularx}
}  
\NewDocumentCommand{\myheaderiii}
{+mmm}
{%
  \begin{tabularx}{\textwidth}{@{}X@{}C@{}R@{}}#1&#2&#3\end{tabularx}
}  
\NewDocumentCommand{\chapterMark}{}{UNDEFINED!}
\NewDocumentCommand{\setchapterMark}{m}
{%
  \RenewDocumentCommand{\chapterMark}{}{#1}
}
\NewDocumentCommand{\newChapter}{mmO{}}
{%
  \setchapterMark{#2}
  \chapter{#1}
  \thispagestyle{specialii}
  {SOME CUST CHAP. FORMAT%THAT NO EXISTING PACKAGE CAN EASILY MAKE
  \textless#1\textgreater}
}

%-----------------------------------------------------------------
\newpagestyle{specialii}
{
  \setfoot{%
    \myheaderii{\thisdoc}{\thepage/\pageref{LastPage}}%
  }{}{}
}
\newpagestyle{specialiii}
{
  \setfoot{%
    \myheaderiii{\thisdoc}{\chapterMark}{\thepage/\pageref{LastPage}}%
  }{}{}
}

\begin{document}%-------------------------------------------------
\thispagestyle{empty}
\begin{titlepage}\vspace*{\fill}\huge\begin{center}\thisdoc\end{center}\vspace*{\fill}\end{titlepage}
\tableofcontents
\pagestyle{specialiii}
\newChapter{Writing}{\faPencil}

Let's make that span\newpage another page

\newChapter{Fonts}{\faFont}

Fonts are great.

\end{document} 

EDIT on 10/10:

• Replaced the crude modification on internals of \chapter with one using \patchcmd.

• Added \usepackage[french]{babel}, which causes problems (french added to the chapter title), hence

• Unchecked this answer