I suggest using xltabular
, a combination of tabularx
and longtable
packages, instead of longtable
:
\documentclass[arpaper]{report}
\usepackage{geometry}
\usepackage[T1]{fontenc}
\usepackage[table]{xcolor}
\usepackage{ragged2e}
\usepackage{rotating}
\usepackage{makecell, multirow, xltabular}
\renewcommand\theadfont{\bfseries\small}
\newcolumntype{R}{>{\RaggedRight}X}
\usepackage{pdflscape}
\usepackage{lipsum}
\begin{document}
\begin{landscape}
\setlength\LTcapwidth\linewidth
\settowidth\rotheadsize{\textbf{Independent}}
\setlength\extrarowheight{2pt}
\begin{xltabular}{\linewidth}{| c | *{6}{R|}}
\caption[blabla]{\lipsum*[66]}
\label{tab:primer} \\
\hline
\rowcolor{lightgray}
\thead{Category}
& \thead{Tehnique}
& \thead{Description}
& \thead{Dis)advantages}
& \thead{Example}
& \thead{xxxxx}
& \thead{xxxxxx} \\
\hline
\endfirsthead
\caption[]{\lipsum*[66] (Cont.)} \\
\hline
\rowcolor{lightgray}
\thead{xxxxx}
& \thead{xxxxx}
& \thead{xxxxxx}
& \thead{xxxxx}
& \thead{xxxxxx}
& \thead{xxxxx}
& \thead{xxxxxx} \\
\hline
\endhead
\hline
\multicolumn{7}{r}{\small\emph{Continued on next page}} \\
\endfoot
\hline
\endlastfoot
\multirow{3}{*}[3ex]{\rothead{Direct}}
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx \\
\hline
\multirow{6}{*}[3ex]{\rothead{Indirect}}
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx \\
\hline
\multirow{6}{*}[1ex]{\rothead{Independent}}
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx \\
\hline
\multirow{6}{*}[3ex]{\rothead{Indirect}}
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx \\
\multirow{6}{*}[2ex]{\rothead{Direct}}
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx \\
\hline
\multirow{6}{*}[3ex]{\rothead{Indirect}}
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx \\
\end{xltabular}
\end{landscape}
\end{document}

Addendum:
Regarding your comment(s): at width of columns you are restricted with text width. So you cannot define any width of column. Some reasonable measure which you can take are:
- use smaller font, for example
\small
or \footnotesize
)I don't recommend smaller)
- change ratio between column width
- for text in the first column header use use abbreviation, for example
C.
\documentclass[arpaper]{report}
\usepackage{geometry}
\usepackage[T1]{fontenc}
\usepackage[table]{xcolor}
\usepackage{ragged2e}
\usepackage{rotating}
\usepackage{makecell, multirow, xltabular}
\renewcommand\theadfont{\bfseries\small}
\newcolumntype{C}{>{\Centering}X}
\newcolumntype{R}{>{\RaggedRight}X}
\usepackage{pdflscape}
\usepackage{lipsum}
\begin{document}
\begin{landscape}
\setlength\LTcapwidth\linewidth
\settowidth\rotheadsize{\textbf{Independent}}
\setlength\extrarowheight{2pt}
\setlength\tabcolsep{3pt}
\small
\begin{xltabular}{\linewidth}{| >{\hsize=0.2\hsize}C |
*{2}{>{\hsize=0.7\hsize}R|}
*{4}{>{\hsize=1.10\hsize}R|}}
\caption[blabla]{\lipsum*[66]}
\label{tab:primer} \\
\hline
\rowcolor{lightgray}
\thead{C.}
& \thead{Tehnique}
& \thead{Description}
& \thead{Dis)advantages}
& \thead{Example}
& \thead{xxxxx}
& \thead{xxxxxx} \\
\hline
\endfirsthead
\caption[]{\lipsum*[66] (Cont.)} \\
\hline
\rowcolor{lightgray}
\thead{xxxxx}
& \thead{xxxxx}
& \thead{xxxxxx}
& \thead{xxxxx}
& \thead{xxxxxx}
& \thead{xxxxx}
& \thead{xxxxxx} \\
\hline
\endhead
\hline
\multicolumn{7}{r}{\small\emph{Continued on next page}} \\
\endfoot
\hline
\endlastfoot
\multirow{11}{*}{\rothead{Direct}}
& \lipsum[1][1-1]
& \lipsum[1][1-2]
& \lipsum[1][1-3]
& \lipsum[1][1-4]Method
& \lipsum[66]
& \lipsum[66] \\
\hline
\multirow{6}{*}[4ex]{\rothead{Indirect}}
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx \\
\hline
\multirow{6}{*}[1ex]{\rothead{Independent}}
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx \\
\hline
\multirow{6}{*}[3ex]{\rothead{Indirect}}
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx \\
\hline
\multirow{6}{*}[2ex]{\rothead{Direct}}
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx \\
\end{xltabular}
\end{landscape}
\end{document}

If you able to use (recent version of) tabularray package, than the table code is far simpler and not require manual tweaking of \multirow cells since in your table are not needed. For example:
\documentclass[arpaper]{report}
\usepackage{geometry}
\usepackage[T1]{fontenc}
\usepackage{microtype}
\usepackage{xcolor}
\usepackage{rotating}
\usepackage{makecell}
\usepackage{tabularray}
\usepackage{pdflscape}
\usepackage{lipsum}
\begin{document}
\begin{landscape}
\begin{longtblr}[
caption = {\lipsum*[66]},
entry = {blabla},
label = {tab:primer}
]{hlines, vlines,
colspec = {Q[c,m]
*{2}{X[0.70, l,h]}
*{4}{X[1.15, l,h]}
},
colsep = 3pt,
cell{2-Z}{1} = {cmd=\rotcell[cc]},
row{1} = {font=\footnotesize\bfseries, c, bg=lightgray},
rowhead = 1,
vspan=even}
%%%% COLUMNS HEADERS
Cat.
& Tehnique
& Description
& advantages
& Example
& xxxxx
& xxxxxx \\
%%%% BODY
Direct
& \lipsum[1][1-1]
& \lipsum[1][1-2]
& \lipsum[1][1-3]
& \lipsum[1][1-4]
& \lipsum[66]
& \lipsum[66] \\
Indirect
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx \\
Independent
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx \\
Indirect
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx
& Method
& x & Advantages: \par
(1) xxx \par
(2) xxx \par
Disadvantages: \par
(1) xxx \par
(2) xxx \\
\end{longtblr}
\end{landscape}
\end{document}

\begin{document}
?) and it's fragmented into several parts. Keep it in one piece so that people can copy-paste it easily.p{0.22\textwidth}
alsonever usecenter
withlongtable
\textwidth
and so fit on the page. That is simple arithmetic, nothing particularly aboutlongtable