I would write your table on the following way:
\documentclass{article}
\usepackage{geometry}
\usepackage{rotating}
\usepackage{makecell}
\renewcommand\theadfont{}
\usepackage{tabularray}
\UseTblrLibrary{siunitx}
\SetTblrStyle{caption-tag}{font=\small\bfseries}
\SetTblrStyle{caption-text}{font=\small}
\begin{document}
\begin{talltblr}[
caption = {Ergebnisse der Gitterschnittprüfung der bei \qty{40}{\degreeCelsius} gehärteten Proben PUA 2403 X, -Y und -Z},
label = {tblr:GT 40 grad},
]{hlines, vlines,
colspec = {X[0.5, c,m] X[1.2, c,m] *{6}{X[c,m]} },
colsep = 3pt,
cell{2,10}{1} = {r=4}{cmd=\settowidth\rotheadsize{2403 X}\rothead},
cell{6}{1} = {r=4}{cmd=\settowidth\rotheadsize{2403 X}\rothead, h},
row{1} = {font=\small\bfseries},
}
PUA Nr. & Substrat & Blech Nr. & GT Nr. 1 & GT Nr. 2 & GT Nr. 3 & Mittelwert & Stabw. S \
2403 X & Oxsilan & 4 & 0 & 0 & 0 & 0 & 0 \
& Fe Phosphatiert
& 5 & 0 & 0 & 0 & 0 & 0 \
& Zn Phosphatiert
& 1 & 0 & 0 & 0 & 0 & 0 \
& Gestrahltes Aluminium
& 1 & 0 & 0 & 0 & 0 & 0 \
2403 Y & Oxsilan & 3 & 0 & 0 & 0 & 0 & 0 \
& Fe Phosphatiert
& 4 & 0 & 0 & 0 & 0 & 0 \
& Zn Phosphatiert
& 2 & 0 & 0 & 0 & 0 & 0 \
& Gestrahltes Aluminium
& 1 & 0 & 0 & 0 & 0 & 0 \
2403 Z & Oxsilan & 1 & 0 & 0 & 0 & 0 & 0 \
& Fe Phosphatiert
& 4 & 0 & 0 & 0 & 0 & 0 \
& Zn Phosphatiert
& 2 & 0 & 0 & 0 & 0 & 0 \
& Gestrahltes Aluminium
& 1 & 0 & 0 & 0 & 0 & 0 \
\end{talltblr}
\end{document}
Main differences to your code snippet are:
- for table is used
talltblr table (since it is not evident that this table spans more pages)
- last six columns have equal width, the first and second column width fare narrower and wider respectively
- for rotate cell content are used
rotating and makecell package

Unfortunately your page layout is unknown. In the case, that your table should be narrower, for example with width of default text block width of the article document class, that you should change MWE and \talltblr preamble to:
\documentclass{article}
%\usepackage{geometry}
\usepackage{rotating}
\usepackage{makecell}
\renewcommand\theadfont{}
\usepackage{tabularray}
\UseTblrLibrary{siunitx}
\SetTblrStyle{caption-tag}{font=\small\bfseries}
\SetTblrStyle{caption-text}{font=\small}
\begin{document}
\begin{talltblr}[
caption = {Ergebnisse der Gitterschnittprüfung der bei \qty{40}{\degreeCelsius} gehärteten Proben PUA 2403 X, -Y und -Z},
label = {tblr:GT 40 grad},
]{hlines, vlines,
colspec = {X[0.7, c,m] X[1.7, c,m] *{6}{X[c,m, cmd=\hspace{0pt}]} },
colsep = 3pt,
cell{2,10}{1} = {r=4}{cmd=\settowidth\rotheadsize{2403 X}\rothead},
cell{6}{1} = {r=4}{cmd=\settowidth\rotheadsize{2403 X}\rothead, h},
row{1} = {font=\small\bfseries},
}
and resulted table will be:

BTW, desired change of your table design make it (according to my opinion) ugly.