Arrow between lines in the align environment

Just for completeness: with tikzmark, which you are using, there is no problem in completing the code to give

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{tikzmark}
\begin{document}
\begin{align*}
abc &= def + def + def + def \tikzmarknode{a}{}\\
    &= ghi + ghi \tikzmarknode{b}{}\\
    &= jkl\tikzmarknode{c}{}\\
    &= mno + mno\tikzmarknode{d}{}
\end{align*}
\begin{tikzpicture}[remember picture, overlay,font=\footnotesize]
\draw[-latex] ([xshift=2mm]a.east) coordinate (r)
    to [out=0,in=0,looseness=2] 
     node[midway,anchor=west,xshift=3mm] {definition}
    (b -| r);
\draw[-latex] (c-|r)
    to [out=0,in=0,looseness=2] 
     node[midway,anchor=west,xshift=3mm] {blablabla}
    (d-|r);
\end{tikzpicture}
\end{document}

This can be done using the witharrows package.

Here is the code:

\documentclass{article}

\usepackage{witharrows}

\begin{document}

\[
\begin{WithArrows}
abc &= def+def+def+def \Arrow[tikz={font={\mdseries}}]{definition} \\
 &= hgi+hgi\\
 &= jkl\Arrow[tikz={font={\mdseries}}]{blablabla}\\
 &=mno+mno
\end{WithArrows}
\]

\end{document}

Note that you may have to compile twice when using witharrows.

Go to the package documentation at CTAN to see the many options for usage including fonts, arrowheads, curvature, color, etc.