Usage of different headings with fancyhdr in a thesis

I have been trying to insert different headings to my thesis.

The requirement is to show the frontmatter with Roman letters and the name of the section or chapter. The heading of the main part shall show the number of the chapter, the chapter title and the page number. The backmatter again shall show the same as the main part, thus the number of the chapter, title and page number. Sounds pretty easy. However, I have my difficulties.

I have tried the following code. It works fine for the frontmatter. The main and backmatter do not show the expected result. In the main matter, the number of the chapter is not shown and on the "rightmark" page, the section is shown instead of the chapter details.

Same for the backmatter. It shows the title twice in the header, but not the chapter number or the page number.

Attached you will find the equivalent code.

usepackage{fancyhdr}
pagestyle{fancy}
fancypagestyle{main}{%
fancyhf{}
%fancyhead[EL,OR]{textsl{rightmark}}
%fancyhead[OL,ER]{textsl{rightmark}}
fancyhead[EL]{thepage} 
fancyhead[ER]{leftmark}
fancyhead[OL]{leftmark}
fancyhead[OR]{thepage}
renewcommand{chaptermark}[1]{%
     markboth{thechapter{} #1}{thechapter{}#1}}
}
fancypagestyle{plain}{%
fancyhf{}
fancyhead[L]{nouppercase{rightmark}}
fancyhead[R]{nouppercase{leftmark}}
fancyhead[EL]{thepage} 
fancyhead[OR]{thepage}

renewcommand{chaptermark}[1]{%
    markboth{#1}{#1}}
}
fancypagestyle{back}{%
fancyhf{}
fancyhead[EL]{thepage} 
fancyhead[ER]{leftmark}
fancyhead[OL]{leftmark}
fancyhead[OR]{thepage}

renewcommand{chaptermark}[1]{
    markboth{thechapter{} #1}{thechapter{}#1}}
}
apptofrontmatter{pagestyle{plain}}
apptomainmatter{pagestyle{main}}
apptobackmatter{pagestyle{back}}



%**Another way of doing it is the following:** 
documentclass[12pt,twoside]{book}
usepackage[utf8]{inputenc}
usepackage{graphicx}
usepackage[a4paper,width=150mm, top=25mm, bottom=25mm, bindingoffset=6mm]{geometry}
usepackage[ngerman]{babel}
usepackage[T1]{fontenc}
usepackage{fontspec}
%usepackage[scaled]{uarial}
setsansfont{Arial}
%usepackage{lmodern}
usepackage{amsmath}
usepackage{siunitx}
usepackage{amsfonts}
usepackage{amssymb}
usepackage[]{unicode-math}
setmainfont[Mapping=tex-text,Numbers={OldStyle,Proportional},Ligatures={TeX, Common}]{TeX Gyre Termes}
setmonofont[Mapping=tex-text,Scale=0.8]{TeX Gyre Cursor}
setsansfont[Mapping=tex-text,Numbers=OldStyle,Ligatures=Common]{TeX Gyre Heros}
setmathfont[math-style=ISO,bold-style=ISO,vargreek-shape=TeX]{TeX Gyre Pagella Math}
usepackage{makeidx}
usepackage{multirow}
usepackage{caption}
usepackage{subcaption}
usepackage{fancyhdr}
setlength{headheight}{15pt}
fancypagestyle{plain}{%
fancyhf{}
fancyhead[L]{nouppercase{leftmark}}
fancyhead[R]{nouppercase{leftmark}}
fancyhead[EL]{thepage} 
fancyhead[OR]{thepage}
renewcommand{chaptermark}[1]{%

    markboth{#1}{#1}}
}
%fancypagestyle{main}
pagestyle{fancy}
fancyhf{}
renewcommand{chaptermark}[1]{ 
    markboth{thechapter{} #1} {} }

fancyhead[LE,RO]{thepage}
fancyhead[RE]{nouppercase{leftmark} }
fancyhead[LO]{nouppercase{leftmark} 
}

graphicspath{images/}
usepackage[style=alphabetic]{biblatex}
addbibresource[style=alphabetic]{references.bib}
usepackage{setspace}
setstretch{1.1}
usepackage{acronym}

parindent0cm
renewcommand*{familydefault}{sfdefault}


makeatletter 
% Define frontmatter, mainmatter and backmatter if not defined 
@ifundefined{frontmatter}{% 
    newcommand{frontmatter}{% 
       %In Roemischen Buchstaben nummerieren (i, ii, iii) 
       pagenumbering{roman} 
    } 
}{} 
@ifundefined{mainmatter}{% 
    % scrpage2 benoetigt den folgenden switch 
    % wenn mainmatter definiert ist. 
%    newifif@mainmatter@mainmattertrue 
    newcommand{mainmatter}{% 
       % -- Seitennummerierung auf Arabische Zahlen zuruecksetzen (1,2,3) 
       pagenumbering{arabic}% 
       setcounter{page}{1}% 
    } 
}{} 
@ifundefined{backmatter}{% 
%    newifif@backmatter@backmattertrue 
    newcommand{backmatter}{ %
       %In Roemischen Buchstaben nummerieren (i, ii, iii) 
       pagenumbering{roman} 
    } 
}{} 
makeatother 

begin{document}
{sffamily
%maketitle
begin{titlepage}
   begin{center}
       vspace*{2cm}

begin{Huge}
{xx}
end{Huge}
       
       vspace{0.5cm}
        {Large xx}
            
       vspace{1cm}
textbf{xx}
        vspace{1cm}
zxx 

        vspace{1cm}
textbf{xx}

        vspace{0.5cm}
textbf{(xx)}
        
vspace{1cm}
          
                             
       von xx
       vspace{0.5cm}
xx 
        vspace{0.5cm}
xx
        vspace{0.5cm}
xx
        vspace{1.5cm}
xx: 
        vspace{0.5cm}
xx
        vspace{0.5cm}
xx
        vspace{1cm}
xx     
%       includegraphics[width=0.4textwidth]{university}
           
            
   end{center}
end{titlepage}
thispagestyle{empty}
frontmatter 
pagestyle{plain}

chapter{Vorwort}
chapter{Danksagung}
xx
chapter{Abstrakt}
xx
chapter{Abstract}
xx
chapter{Abkürzungsverzeichnis}
begin{acronym}[SEPSEP]
acro{abb}[Abb.]{Abbildung}
end{acronym}
newpage
tableofcontents
listoffigures
listoftables
mainmatter
pagestyle{fancy}
chapter{Einleitung}
xx

chapter{ssssssssss}
xx

chapter{eeeeeeeeee}
cccc

backmatter

appendix
chapter{Anhang}
ffffffffff
}

end{document}