Best practice for including standalone commented equations with tikz

Question

I'm writing a document where there are some equations which I'm commenting with hf-tikz package. It needs two LaTeX compilations in order to get the right coordinates and to draw everything in the right place. Looking at How to make a standalone document with one equation? I've been able to build a standalone file with my equation. But I don't know what's the best way to include it in a main document.

Besides, I wonder if it's even possible to keep fonts and sizes, so the inserted equation looks like the main document. Or is the standalone equation always included as an image?

Here is an example of one of my standalone equations:

\documentclass[preview]{standalone}
\usepackage[utf8]{inputenc}

\usepackage{amsmath,amsfonts,amssymb} 
\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage[customcolors]{hf-tikz}

\begin{document}
\vspace*{10mm}
\begin{equation*}
\begin{split}
 4x^2 + 12x + 9 & = \tikzmarkin{a1}(0.05,-0.2)(-0.05,0.4)(2x)^2 \tikzmarkend{a1} + 2\cdot \tikzmarkin{a2}(0.05,-0.2)(-0.05,0.4)(2x)\tikzmarkend{a2} \cdot \tikzmarkin{a3}(0.05,-0.2)(-0.05,0.4)3\tikzmarkend{a3} + \tikzmarkin{a4}(0.05,-0.2)(-0.05,0.4)3^2\tikzmarkend{a4} \\[6ex]
 & = (\,\, \tikzmarkin{a9}2x \tikzmarkend{a9}+ \tikzmarkin{a10} 3\tikzmarkend{a10} \,\,)^2
\end{split}
\end{equation*}

\begin{tikzpicture}[remember picture,overlay]% adjust the shift from "col" to move the position of the annotation
\coordinate (a5) at ($(a1)+(0.5,0.6)$);
\node[align=left,above] at (a5) {\large$a^2$};
\path[-stealth,red,draw] (a5) -- ($(a1)+(0.5,0)$);

\coordinate (a6) at ($(a2)+(0.4,0.6)$);
\node[align=left,above] at (a6) {\large$a$};
\path[-stealth,red,draw] (a6) -- ($(a2)+(0.4,0)$);

\coordinate (a7) at ($(a3)+(0.15,0.6)$);
\node[align=left,above] at (a7) {\large$b$};
\path[-stealth,red,draw] (a7) -- ($(a3)+(0.15,0)$);

\coordinate (a8) at ($(a4)+(0.2,0.6)$);
\node[align=left,above] at (a8) {\large$b^2$};
\path[-stealth,red,draw] (a8) -- ($(a4)+(0.2,0)$);

\coordinate (a11) at ($(a9)+(0.3,0.3)$);
\node[align=left,above] at (a11) {\large$a$};
\path[-stealth,red,draw] (a11) -- ($(a9)+(0.3,0)$);

\coordinate (a12) at ($(a10)+(0.2,0.3)$);
\node[align=left,above] at (a12) {\large$b$};
\path[-stealth,red,draw] (a12) -- ($(a10)+(0.2,0)$);
\end{tikzpicture}

\end{document}

UPDATE TO CLARIFY I don't want the standalone file to be included as raw tex, since it's possible I reuse some of the coordinates names in other equations. The following doesn't work, as I'm not loading the required packages in the main preamble.

\documentclass{article}
\usepackage{standalone}
\begin{document}
\includestandalone{my-standalone-file}
\end{document}

Answer 1

The standalone package allows for automated importing the preambles of the sub-files to the main document using the subpreambles option. However, if you have multiple sub-files with clashing preambles you will run into problems.

If you want to have a common preamble in the main document and all sub-files (or just all sub-files) then just place the common preamble code into a separate .tex file and \input this one in all files in the preamble.