Multlined in align enviromnent: alignment of "RHS =" and equation numbering

Question

I'm using multlined (from mathtools) inside align to show a chain of equivalences between expressions, some of which may not fit in a line. I would like to align the equation numbering at the bottom (or centre) of the lines that are part of the same expression. Edited: Because I have other multline elsewhere, for consistency, I would like the broken lines to be aligned as if they were standalone multline equations, i.e. first line left, middle lines centre, last line right.

While an answer to Multlined in align enviromnent: alignment of = and equation numbering provides a solution for the lines after the first one, that solution does not work for the first line because the RHS of the first equality is misaligned.

\documentclass{article}
\usepackage[width=7.00cm, height=17.00cm]{geometry}
\usepackage{amsmath,mathtools}

\begin{document}

    \begin{align}
        y &  \begin{multlined}[b]
        = a+b+{} \\
        c+d+{} \\
        e+f+g+h+i+j+k+l
        \end{multlined} \\
        &= a+b+c
    \end{align}

\end{document}

The only solution that I found involves the use of phantom and is a bit hacky. I wonder if a better solution (Edited: which does not require phantoms or manual spacing) exists.

Answer 1

try this, using \split within the align:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

    \begin{align}
      \begin{split}
        y &= a+b+{} \\
          &\qquad c+d+e
      \end{split} \\
        &= a+b+c
    \end{align}

\end{document}

Answer 2

My solution involves changing the alignment point of align to the beginning of the line (rather than at the equal sign) and then using a phantom to add an empty space corresponding to the RHS to all the subsequent lines.

\documentclass{article}
\usepackage[width=7.00cm, height=11.00cm]{geometry}
\usepackage{amsmath,mathtools}

\begin{document}

    \begin{align}
        &\begin{multlined}[b]
        y = a+b+{} \\
        \phantom{y=}c+d+{}\\
        \phantom{y=}e+f+g+h+i+j+k+l
        \end{multlined} \\
        &\phantom{y} = a+b+c
    \end{align}

\end{document}

This gives the desired behaviour (Eqs 1 and 2 in the question).

As I try to use phantoms as last resort, I wonder if someone can come up with a better solution.