Here's another solution. It's quite similar to @Zarko's in the use of the S
column type for the six numeric columns, to align all numbers on their respective decimal markers. It differs from @Zarko's solution in (a) the use of a tabular*
environment, (b) the provision of more visual structure to the header material, and (c) no reliance on the threeparttable
package and its eponymous environment.
For sure, your table isn't all that wide. No need to take any heroic measures to make the table fit inside the width of the textblock.

\documentclass[%
paper=A4,pagesize=auto,BCOR=1cm,
DIV=classic,fontsize=12pt,headings=normal,
bibliography=totoc,titlepage=on,
captions=tableheading,captions=oneline,
numbers=noendperiod,toc=listof,draft=false
]{scrartcl}
\usepackage{calc,pifont,ifthen,enumerate}
\usepackage[round]{natbib}
\usepackage[english,ngerman]{babel}
\usepackage[centertags,fleqn]{amsmath}
\usepackage{textcomp}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[bitstream-charter]{mathdesign}
\usepackage{sourcesanspro,microtype,srcltx,fancyvrb}
\usepackage[final]{graphicx}
\usepackage{color,caption,siunitx,booktabs}
\usepackage[right, official]{eurosym}
\usepackage{scrlayer-scrpage,graphicx,adjustbox,tabularx}
\usepackage{hyperref} % load this package last!
\begin{document}
\begin{table}[htbp]
\small
\caption{Effects My-Modell} \label{Tab3.4}
\sisetup{table-format=-1.6,group-digits=false}
\setlength\tabcolsep{0pt} % let LaTeX figure out intercolumn whitespace
\begin{tabular*}{\textwidth}{ @{\extracolsep{\fill}}
S[table-format=2.1] *{6}{S} }
\toprule
{$\eta$} &
\multicolumn{6}{c}{$\sigma$} \\
\cmidrule{2-7}
& 0.0036 & 0.0072 & 0.0108 & 0.0144 & 0.0180 & 0.0216 \\
\midrule
1.1& 0.001317& 0.005266& 0.011850& 0.021067& 0.032920& 0.047409\\
& 0.003601& 0.014406& 0.032416& 0.057637& 0.090073& 0.129734\\
2.0&-0.001071&-0.004285&-0.009640&-0.017137&-0.026774&-0.038549\\
& 0.002354& 0.009416& 0.021188& 0.037674& 0.058878& 0.084807\\
3.0&-0.002988&-0.011950&-0.026882&-0.047775&-0.074619&-0.107398\\
& 0.002352& 0.009408& 0.021171& 0.037646& 0.058841& 0.084764\\
4.0&-0.004882&-0.019524&-0.043908&-0.078005&-0.121777&-0.175171\\
& 0.002790& 0.011162& 0.025121& 0.044676& 0.069842& 0.100635\\
5.0&-0.006818&-0.027258&-0.061278&-0.108810&-0.169756&-0.243994\\
& 0.003604& 0.014421& 0.032462& 0.057748& 0.090304& 0.130167\\
6.0&-0.008789&-0.035126&-0.078931&-0.140064&-0.218335&-0.313502\\
& 0.004802& 0.019217& 0.043270& 0.077002& 0.120473& 0.173758\\
7.0&-0.010781&-0.043075&-0.096736&-0.171524&-0.267108&-0.383068\\
& 0.006431& 0.025742& 0.057985& 0.103248& 0.161654& 0.233365\\
8.0&-0.012780&-0.051042&-0.114552&-0.202928&-0.315641&-0.452031\\
& 0.008524& 0.034132& 0.076929& 0.137092& 0.214872& 0.310596\\
9.0&-0.014772&-0.058971&-0.132248&-0.234028&-0.315641&-0.519781\\
& 0.011043& 0.044237& 0.099781& 0.178012& 0.279410& 0.404607\\
10.0&-0.016747&-0.066821&-0.149723&-0.264610&-0.410474&-0.585862\\
& 0.014061& 0.056362& 0.127264& 0.227377& 0.357582& 0.519048\\
\bottomrule
\end{tabular*}
\smallskip
\footnotesize\textbf{Notes:} Effects in Percent. Here is an explanation
of the table, the parameters, the measurement etc. It's quite long
and also has to fit inside the width of the textblock.
\end{table}
\end{document}
Best Answer
If it is OK for you to have one page rotated as a landscape where to have only the table, try the
lscape
package:See also this question: How to change certain pages into landscape/portrait mode