两个多行公式的等号如何对齐？

\documentclass{book}
\usepackage[margin=1cm]{geometry}
\usepackage{showframe}
\usepackage{amsmath}
\counterwithin{equation}{section}
\usepackage{tabularray}
\UseTblrLibrary{counter}
\begin{document}
\chapter{title}
\section{title}
\begin{equation*}
\begin{tblr}
{
colspec    = {X[c,m]Q[r,m]Q[l,m]X[r,m]},
columns    = {colsep=0pt},
column{Z}  = {mode=text},
row{5}     = {abovesep+=\abovedisplayskip},
cell{1}{Z} = {r=4}{preto=(\stepcounter{equation}\theequation)},
cell{5}{Z} = {r=7}{preto=(\stepcounter{equation}\theequation)},
column{3}  = {preto=\mbox{}}
}
& a+a+a+a+a+a+a+a+a+a & =b+b+b       & \\
&                     & =c+c         & \\
&                     & =c+c         & \\
&                     & =c+c         & \\
& a+a+a               & =b+b+b+b+b+b & \\
&                     & =c+c         & \\
&                     & =c+c         & \\
&                     & =c+c         & \\
&                     & =c+c         & \\
&                     & =c+c         & \\
&                     & =c+c         & \\
\end{tblr}
\end{equation*}
\end{document}