How to place the chapter number behind the chapter title in KOMA script

You mean like this?

That doesn't look good, does it?

At the very least, I would do something like this:

\renewcommand*{\chapterformat}{%
  \rlap{\makebox[\linewidth][r]{\colorbox{ltGray}{\fontsize{60}{68}\selectfont\color{Gray}

\thechapter\autodot}}}}
\addtokomafont{chapter}{%
  \renewcommand*{\raggedsection}{\raggedleft\rightskip1.2em\hfill}}

Disclaimer: I'm not a designer!