tikz to eps quality loss

Question

I generated an image with tikz which consists in an external image and a series of labels. I tried to convert it in .eps format by following the procedure presented here Export eps figures from TikZ. As result the .pdf image is good while I have a huge quality degradation in the .eps file.

In the preamble I included the following lines

\usetikzlibrary{external}
\tikzset{external/system call={pdflatex \tikzexternalcheckshellescape -halt-on-error 
-interaction=batchmode -jobname "\image" "\texsource";
pdftops -eps "\image".pdf;
ps2eps "\image.ps"}}
\tikzexternalize

and then I run the command

latex --shell-escape

Here a complete example of the code

\documentclass[]{article}
\usepackage{amsmath,bm}
\usepackage{amssymb}
\usepackage{esint}
\usepackage{tabularx}
\usepackage{graphicx}
\usepackage{tikz}
\usepackage{color}
\usepackage{tikzscale}
\usetikzlibrary{external}
\tikzset{external/system call={pdflatex \tikzexternalcheckshellescape -halt-on-error 
-interaction=batchmode -jobname "\image" "\texsource";
pdftops -eps "\image".pdf;
ps2eps "\image.ps"}}
\tikzexternalize
\begin{document}
\begin{figure*}
\begin{tikzpicture}
\node {\includegraphics[scale=0.15]{circle}};
  \node {$\mathcal{SSSSSSSS}_1$};
\end{tikzpicture}
\end{figure*}
\end{document}

Original:

And here the .pdf and .eps images