Align itemize in multicol with fraction in displaymode

Question

I'm doing a cheat sheet to try to practice with LaTeX. My problem is that after I use itemize + multicols, it shows as below, must be due to that the fraction is in displaystyle. I want the bullet in the middle to be aligned with the other two on the first line. Can anyone show me how to do it?

\documentclass[a4paper]{article}
\usepackage[a4paper,
            left=2cm,right=2cm,
            top=2cm,bottom=1.5cm]{geometry}
\usepackage{amsmath,amssymb,amsthm}
%\usepackage[utf8]{vietnam}
\usepackage{enumitem}
\usepackage{xcolor}
\usepackage{tcolorbox}
\usepackage{tabularx}
\usepackage{tikz}
\usepackage{multicol}
    \begin{multicols}{3}
        \begin{itemize}[itemsep = 5pt]
            \item $\bar{\bar{z}}=z$
            \item $\overline{z_1\pm\overline{z_2}}=\overline{z_1}\pm \overline{z_2}$
            \item $\overline{\displaystyle\left(\frac{z_1}{z_2}\right)}=\displaystyle\frac{\overline{z_1}}{\overline{z_2}}$
            \item $\overline{z_1\cdot z_2}=\overline{z_1}\cdot\overline{z_2}$
            \item $z\bar{z}=|z|^2=a^2+b^2$
            \item 
        \end{itemize}
    \end{multicols}
\end{document}

Answer 1

Here is a code with tasks. I took the liberty to simplify a bit your preamble:

\documentclass[a4paper]{article}
\usepackage[a4paper, margin=2cm,bottom=1.5cm]{geometry}
\usepackage{amsmath,amssymb,amsthm}
%\usepackage[utf8]{vietnam}
\usepackage{enumitem}
\usepackage{xcolor}
\usepackage{tcolorbox}
\usepackage{tabularx}
\usepackage{tikz}
\usepackage{multicol}
\usepackage{tasks}

\begin{document}

\begin{tasks}[style=itemize](3)
        \task $\bar{\bar{z}}=z$
        \task $\overline{z_1\pm\overline{z_2}}=\overline{z_1}\pm \overline{z_2}$
        \task $\overline{\Bigl(\dfrac{z_1}{z_2}\Bigr)}=\dfrac{\overline{z_1}}{\overline{z_2}}$
        \task $\overline{z_1\cdot z_2}=\overline{z_1}\cdot\overline{z_2}$
        \task $z\bar{z}=|z|^2=a^2+b^2$
        \task
\end{tasks}

\end{document} 

Answer 2

Just for fun:

\documentclass[a4paper]{article}
\usepackage[showframe,
            left=2cm,right=2cm,
            top=2cm,bottom=1.5cm]{geometry}
\usepackage{amsmath,amssymb,amsthm}
%\usepackage[utf8]{vietnam}
%\usepackage{enumitem}
%usepackage{xcolor}
%\usepackage{tcolorbox}
%\usepackage{tabularx}
%\usepackage{tikz}
%\usepackage{multicol}

\newlength{\cellwidth}
\newcommand{\fakeitem}[1]{% text as argument
  \hskip 0pt plus 0.001fil\allowbreak
  \makebox[\dimexpr \leftmargini-\labelsep][r]{\textbullet}%\nobreak
  \hskip\labelsep%\nobreak
  \makebox[\cellwidth][l]{#1}\ignorespaces}

\begin{document}
\medskip\noindent
\setlength{\cellwidth}{\dimexpr \textwidth/3-\leftmargini}% 3 columns
\fakeitem{$\bar{\bar{z}}=z$}
\fakeitem{$\overline{z_1\pm\overline{z_2}}=\overline{z_1}\pm \overline{z_2}$}
\fakeitem{$\overline{\displaystyle\left(\frac{z_1}{z_2}\right)}=\displaystyle\frac{\overline{z_1}}{\overline{z_2}}$}
\fakeitem{$\overline{z_1\cdot z_2}=\overline{z_1}\cdot\overline{z_2}$}
\fakeitem{$z\bar{z}=|z|^2=a^2+b^2$}
\par\medskip
\noindent next line of text
\end{document}