chapter cover page that spans two pages

Question

I am writing up my PhD thesis and I'd like to make a two-page cover page for each chapter, similar to what was done in this question, but I can't get their example to work. (I am using the report class)

I have in mind something like the attached image but I'm comfortable with playing around with the background and layout once I know how to add the extra cover page so the design isn't super important at this stage, just getting the extra page on the left. So far I have only been able to add a single cover chapter page like in this question. I've spent all day trying things and I'm not winning so I'd appreciate any tips!

This is what I have tried so far, but I am getting an extra blank page after following the suggestion by David Carslisle:

\documentclass[12pt, twoside, openright]{report}
\usepackage[a4paper,width=150mm,top=25mm,bottom=25mm]{geometry}
\usepackage{lipsum}
\usepackage[utf8]{inputenc}
\usepackage{xcolor}
\usepackage{hyperref}
\setlength{\parindent}{0pt} % remove indent on new paragraph line 
%%%%%% labels "thumbs" for each chapter: https://tex.stackexchange.com/questions/119880/show-current-chapter-number-on-each-page-margin
\usepackage[height={1.5cm},distance={10mm},topthumbmargin={auto},bottomthumbmargin={auto}]{thumbs}
%%%%%% add a blank page 
\newcommand\blankpage{%
    \null
    \thispagestyle{empty}%
    %\addtocounter{page}%
    \newpage}
%%%%%% https://tex.stackexchange.com/questions/664274/how-do-i-make-it-so-that-every-chapter-starts-at-an-odd-numbered-page-preceded
\def\cleartoeven{\clearpage\if@twoside \ifodd\c@page
     \thispagestyle{plain}\hbox{}\newpage
     \if@twocolumn\hbox{}\newpage\fi\fi\fi}
%%%%%% https://tex.stackexchange.com/questions/89761/chapters-title-page-before-every-chapter-and-every-chapter-with-its-title
\usepackage{titlesec}
\titleclass{\chapter}{page}
\assignpagestyle{\chapter}{empty}
\titleformat{\chapter}
    [display]
    {\centering\huge\bfseries}
    {\chaptername\ \thechapter}
    {0pt}
    {\Large}
    [\clearpage]
\title{title}
\author{author}
\date{ \today}
\begin{document}
\pagenumbering{arabic}
\maketitle
\tableofcontents
\cleartoeven %for the left page
\chapter{Introduction} % for the chapter page on the right
\clearpage %for rest of chapter
\addthumb{Chapter \thechapter}{\Large{\thechapter}}{white}{gray}
\lipsum[]
\end{document}

Answer 1

Without titlesec I think you want

\documentclass[12pt, twoside, openright]{report}
\usepackage[a4paper,width=150mm,top=25mm,bottom=25mm]{geometry}
\usepackage{lipsum}
\usepackage[utf8]{inputenc}
\usepackage{xcolor}
\usepackage{hyperref}
\setlength{\parindent}{0pt} % remove indent on new paragraph line 
%%%%%% labels "thumbs" for each chapter: https://tex.stackexchange.com/questions/119880/show-current-chapter-number-on-each-page-margin
\usepackage[height={1.5cm},distance={10mm},topthumbmargin={auto},bottomthumbmargin={auto}]{thumbs}
%%%%%% add a blank page 
\newcommand\blankpage{%
    \null
    \thispagestyle{empty}%
    %\addtocounter{page}%
    \newpage}
%%%%%% https://tex.stackexchange.com/questions/664274/how-do-i-make-it-so-that-every-chapter-starts-at-an-odd-numbered-page-preceded
\def\cleartoeven{\clearpage\if@twoside \ifodd\c@page
     \thispagestyle{plain}\hbox{}\newpage
     \if@twocolumn\hbox{}\newpage\fi\fi\fi}
%%%%%% https://tex.stackexchange.com/questions/89761/chapters-title-page-before-every-chapter-and-every-chapter-with-its-title
\iffalse
\usepackage{titlesec}
\titleclass{\chapter}{page}
\assignpagestyle{\chapter}{empty}
\titleformat{\chapter}
    [display]
    {\centering\huge\bfseries}
    {\chaptername\ \thechapter}
    {0pt}
    {\Large}
\fi


\title{title}
\author{author}
\date{ \today}
\begin{document}
\pagenumbering{arabic}
\maketitle
\tableofcontents
\cleartoeven %for the left page
stuff here on left
\chapter{Introduction} % for the chapter page on the right
\addthumb{Chapter \thechapter}{\Large{\thechapter}}{white}{gray}
\clearpage
\lipsum[]
\end{document}

with it, I'm not sure: it's inserting a \cleardoublepage somewhere but I can't see why at the moment.