Command for character count including spaces (in overleaf)

Here's one idea, similar to your approach, but using egreg's answer to Read number from file :

\documentclass{article}
\newread\tmp

\newcommand{\quickcharcount}[1]{%
  \immediate\write18{texcount -1 -sum -merge -char #1.tex > #1-chars.sum}%
  \openin\tmp=#1-chars.sum%
  \read\tmp to \thechar%
  \closein\tmp%
}

\newcommand{\quickwordcount}[1]{%
  \immediate\write18{texcount -1 -sum -merge #1.tex > #1-words.sum}%
  \openin\tmp=#1-words.sum%
  \read\tmp to \theword%
  \closein\tmp%
}

\begin{document}
\quickwordcount{main}
\quickcharcount{main}

There are \thechar characters and approximately \theword spaces.
That makes approximately \the\numexpr\theword+\thechar\relax\ characters total.

\end{document}

The \theword and \thechar macros could be extended to include the filename argument if you need to tally up multiple files, but I've kept it fairly simple here.