Several attempts to improve the appearance in a subequation environment

Question

I am trying in every way to find all the possibilities to best center the first equation in a subequations environment. I can't use the option alignat because the colored enumeration (here there is the full code of @Werner An elegant aspect for enumerate the equations of a book)

\documentclass{book}

\usepackage{mathtools}
\usepackage[svgnames, dvipsnames, table, x11names]{xcolor}
\usepackage{pifont}

\definecolor{ocre}{RGB}{243,102,25} 
%----------------------------------------------------------------------------------
%   Change number equations with sections
%----------------------------------------------------------------------------------
\usepackage{chngcntr}
\counterwithin{equation}{section}

\renewcommand{\theequation}{\thesection-\arabic{equation}}
\newcommand{\eqnnumsymbol}{\textcolor{ocre}{\reflectbox{\ding{228}}}}

\makeatletter
\newtagform{bbrackets}% <name>
  [\textbf]% Internal style
  {(}% Left bracket
  {\ifnum\pdfstrcmp{\@currenvir}{equation}=0
     \eqnnumsymbol%
   \else
     \ifnum\pdfstrcmp{\@currenvir}{align}=0
       \eqnnumsymbol%
     \fi
   \fi
   )}% Right bracket
\makeatother
\usetagform{bbrackets}

\begin{document}

See both~\eqref{eq:eqn1} and~\eqref{eq:eqn2}. Also see~\eqref{eq:eqn3}.

\begin{equation}
  \oint \mathbf{E} \cdot d \mathbf{a} = \int \rho \, \mathrm{d}v \label{eq:eqn1}
\end{equation}

\begin{equation}
  \oint \mathbf{B} \cdot d \mathbf{a} =  \mathbf{0} \label{eq:eqn2}
\end{equation}

\begin{align}
  abc \label{eq:eqn3}
\end{align}

\end{document}

disappears in my book.

I tried to reverse the symbols of & or to add it, but nothing to do. Do you have any suggestions? Thank you all.

I am adding the MWE and the screenshot.

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb}
\begin{document}
\begin{subequations}
\begin{align}
\overline {F} &=-\overline{\nabla}U(\bar{r})+ \overline{\nabla} \times \overline{W}(\bar{r}) \label{Undec}\\
U(\bar{r})&=\frac1{4\pi}\int_{\mathcal{V}}\frac{D(\bar{r}')}{R}dV', \quad \overline{W}(\bar{r})=\frac1{4\pi}\int_{\mathcal{V}}\frac{\overline{C}(\bar{r}')}{R}dV' \label{Undec1}
\end{align}
\end{subequations}
\end{document}

ADDENDUM

With gather option I obtain this:

Answer 1

If you don't need the equations to be aligned at the equal signs, then I believe the gather environment does what you want:

\documentclass{article}
\usepackage{amsmath}

% From egreg: <https://tex.stackexchange.com/a/501075/73317>
\newcommand{\diff}{\mathop{}\!\mathrm{d}}

\begin{document}

\begin{subequations}
\begin{gather}
\overline{F} = - \overline{\nabla}U(\bar{r}) +
               \overline{\nabla} \times \overline{W}(\bar{r}) \label{Undec}\\
U(\bar{r}) = \frac{1}{4\pi} \int_{\mathcal{V}} \frac{D(\bar{r}')}{R} \diff V',
\quad
\overline{W}(\bar{r}) =
  \frac{1}{4\pi} \int_{\mathcal{V}} \frac{\overline{C}(\bar{r}')}{R} \diff V'
  \label{Undec1}
\end{gather}
\end{subequations}

\end{document}

Note that I removed the & before the equal signs (no alignment tabs in gather). I also used a \diff command from egreg for the differential.

Here is the same with your red arrows in equation tags. First using the same technique to check the environment name (there is a lot of redundancy, we'll remove it below):

\documentclass{article}
\usepackage{mathtools}
\usepackage{graphicx}
\usepackage{xcolor}
\usepackage{pifont}

% From egreg: <https://tex.stackexchange.com/a/501075/73317>
\newcommand{\diff}{\mathop{}\!\mathrm{d}}

\definecolor{ocre}{RGB}{243,102,25}
\counterwithin{equation}{section}

\renewcommand{\theequation}{\thesection-\arabic{equation}}
\newcommand{\eqnnumsymbol}{\textcolor{ocre}{\reflectbox{\ding{228}}}}

\makeatletter

\newtagform{bbrackets}% <name>
  [\textbf]% Internal style
  {(}% Before
  {\ifnum\pdfstrcmp{\@currenvir}{equation}=0
     \eqnnumsymbol
   \else
     \ifnum\pdfstrcmp{\@currenvir}{align}=0
       \eqnnumsymbol
     \else
       \ifnum\pdfstrcmp{\@currenvir}{gather}=0
         \eqnnumsymbol
       \fi
     \fi
   \fi
   )}% After

\makeatother

\usetagform{bbrackets}

\begin{document}

\section{First section}

\begin{subequations}
\begin{gather}
\overline{F} = - \overline{\nabla}U(\bar{r}) +
               \overline{\nabla} \times \overline{W}(\bar{r}) \label{Undec}\\
U(\bar{r}) = \frac{1}{4\pi} \int_{\mathcal{V}} \frac{D(\bar{r}')}{R} \diff V',
\quad
\overline{W}(\bar{r}) =
  \frac{1}{4\pi} \int_{\mathcal{V}} \frac{\overline{C}(\bar{r}')}{R} \diff V'
  \label{Undec1}
\end{gather}
\end{subequations}

\end{document}

Now, do the environment name check inside \newtagform{bbrackets}[...]{...}{...} in a more elegant way, so that you just have to add the environments in which you want equation tags to have the red arrow to a comma list (see the \forcsvlist{...}{equation, align, gather} line below).

\documentclass{article}
\usepackage{mathtools}
\usepackage{graphicx}
\usepackage{xcolor}
\usepackage{pifont}
\usepackage{etoolbox}

% From egreg: <https://tex.stackexchange.com/a/501075/73317>
\newcommand{\diff}{\mathop{}\!\mathrm{d}}

\definecolor{ocre}{RGB}{243,102,25}

\counterwithin{equation}{section}
\renewcommand{\theequation}{\thesection-\arabic{equation}}
\newcommand{\eqnnumsymbol}{\textcolor{ocre}{\reflectbox{\ding{228}}}}

\makeatletter

% Helper macro used to insert the arrow symbol from pifont if the current
% environment (\@currenvir) is in the list passed to \forcsvlist.
\newcommand*{\@usearrowifsame}[2]{%
  \ifstrequal{#1}{#2}{\eqnnumsymbol\listbreak}{}%
}

\newtagform{bbrackets}% <name>
  [\textbf]% Internal style
  {(}% Before
  {% Insert the red arrow from pifont in the equation tag when we are inside
   % one of the environments listed in the second argument.
   \forcsvlist{\expandafter\@usearrowifsame\expandafter{\@currenvir}}%
              {equation, align, gather}%
   )}% After

\makeatother

\usetagform{bbrackets}

\begin{document}

\section{First section}

\begin{subequations}
\begin{gather}
\overline{F} = - \overline{\nabla}U(\bar{r}) +
               \overline{\nabla} \times \overline{W}(\bar{r}) \label{Undec}\\
U(\bar{r}) = \frac{1}{4\pi} \int_{\mathcal{V}} \frac{D(\bar{r}')}{R} \diff V',
\quad
\overline{W}(\bar{r}) =
  \frac{1}{4\pi} \int_{\mathcal{V}} \frac{\overline{C}(\bar{r}')}{R} \diff V'
  \label{Undec1}
\end{gather}
\end{subequations}

\end{document}