\frac with rounded corners

Question

I am overwriting \frac code and making it longer and thicker. I am successful so far. Thick frac is better for printing, but it seems sharp on the edges. I want to make its edges rounded. Is there anyone who knows how to do that. Thanks for reading and responding.

My code:

\documentclass[9pt]{article}
\usepackage{extsizes}
\usepackage{amsmath}
\usepackage{graphicx}

% \frac overwrite
\newcommand{\thickfrac}[2]{\genfrac{}{}{0.8pt}{0}{#1}{#2}}

\newcommand\bigfrac[3][2.0pt]{%
{\thickfrac{\hspace{#1}#2\hspace{#1}}{\hspace{#1}#3\hspace{#1}}}}


\let\oldfrac\frac
\let\frac\bigfrac
% \frac overwritten

\begin{document}
\thispagestyle{empty}
$$
\frac{\frac{a}{b}}{c} - \frac{d}{\frac{e}{1+\frac{f}{g} } } + \frac{h}{i}
$$ 

\end{document}

Output :

Answer 1

While the OP has indicated that he cannot accept this answer because of the packages it uses, I will present it nonetheless to show a practical application of my roundrule.sty package. It is not found on CTAN, but only at the bottom of this answer: Is there such a thing as a `\mathrule`? (rounded endcaps).

Using that package and this MWE below, I first give the OP's original answer (\thickfracA). I then give my answer that only works in \displaystyle, known as \thickfracB. Finally, I give an answer that works across math styles, as \thickfracC.

\documentclass[9pt]{article}
\usepackage{extsizes}
\usepackage{amsmath}
\usepackage{graphicx}
\usepackage{stackengine,scalerel,roundrule}
\stackMath
% \frac overwrite
\newcommand{\thickfracA}[2]{\genfrac{}{}{0.8pt}{0}{#1}{#2}}
% DISPLAYSTYLE ONLY
\newcommand\thickfracB[2]{%
  \setstackgap{S}{2.5pt}%
  \setbox0=\hbox{\stackon{#1}{#2}}%
  \mkern2.4mu%
  \raisebox{\dimexpr.5\ht\strutbox-.5\dp\strutbox}{\stackunder{%
    \stackon{\roundrule[-.4pt]{\wd0}{.8pt}}{#1}%
  }{%
    #2%
  }}%
  \mkern2.4mu%
}
% ANY MATHSTYLE
\newcommand\thickfracC[2]{\ThisStyle{%
  \setbox1=\hbox{$\SavedStyle($}%
  \setstackgap{S}{2.5\LMpt}%
  \setbox0=\hbox{\stackon{\SavedStyle#1}{\SavedStyle#2}}%
  \mkern2.4mu%
  \raisebox{\dimexpr.5\ht1-.5\dp1}{\stackunder{%
    \stackon{\roundrule[-.4\LMpt]{\wd0}{.8\LMpt}}{\SavedStyle#1}%
  }{%
    \SavedStyle#2%
  }}%
  \mkern2.4mu%
}}

\newcommand\bigfrac[3][2.0pt]{%
{\thickfrac{\hspace{#1}#2\hspace{#1}}{\hspace{#1}#3\hspace{#1}}}}

\let\oldfrac\frac
\let\frac\bigfrac
% \frac overwritten
\begin{document}
\centering
ORIGINAL\let\thickfrac\thickfracA
\thispagestyle{empty}
\[
x =\frac{\frac{a}{b}}{c} - \frac{d}{\frac{e}{1+\frac{f}{g} } } + \frac{h}{i}
\]
displaystyle ONLY\let\thickfrac\thickfracB
\[
x =\frac{\frac{a}{b}}{c} - \frac{d}{\frac{e}{1+\frac{f}{g} } } + \frac{h}{i}
\]
ANY MATHSTYLE\let\thickfrac\thickfracC
\[
\scriptstyle
x =\frac{\frac{a}{b}}{c} - \frac{d}{\frac{e}{1+\frac{f}{g} } } + \frac{h}{i}
\]
\end{document}

Here is the zoom on the \displaystyle answer.