standalone: Outsourcing TikZ while relaxing predefined lengths

Question

I have one main document and two standalone files of tikz pics as given in this answer. I need to have \newlength in the main document and all the standalone files in order to be able to run any document on its own without the need to change the main document structure every single time.

However, relaxing lengths as shown in this answer doesn't work for me. So, how can both cases be integrated to work flawlessly?

Main Document

\documentclass{article}
\usepackage{tikz}
\usepackage[subpreambles=true]{standalone}% I need subpreambles to be true
\begin{document}

% I need to keep the following definitions
\newlength{\mylength}
\setlength{\mylength}{1cm}
\let\mylength\relax

\begin{figure}
  \includestandalone[width=\textwidth]{myfrsttikz}
  \caption{My First TikZ picture}
\end{figure}

\begin{figure}
    \includestandalone[width=\textwidth]{mysecondtikz}
    \caption{My Second TikZ picture}
\end{figure}
\end{document}

First Standalone TikZ File

% myfrsttikz.tex %

\documentclass{standalone}
\newlength{\mylength}
\setlength{\mylength}{2cm}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \draw (0,0) -- (\mylength,\mylength);
\end{tikzpicture}
\end{document}

Second Standalone TikZ File

% mysecondtikz.tex %
\documentclass{standalone}
\newlength{\mylength}
\setlength{\mylength}{5cm}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \draw (0,0) -- (\mylength,\mylength);
\end{tikzpicture}
\end{document}

Answer 1

The easiest way to transmit definitions from the main source to other files is to use filecontents and \input. \includestandalone seems like an unnecessary complication.

Main source:

\begin{filecontents}{data}
\newlength{\mylength}
\setlength{\mylength}{1cm}
\end{filecontents}
%
\documentclass{article}
\usepackage{tikz}
%\usepackage[subpreambles=true]{standalone}% I need subpreambles to be true
\begin{document}

\begin{figure}
  \includegraphics[width=\textwidth]{test5}% renamed
  \caption{My First TikZ picture}
\end{figure}

\end{document}

First standalone:

\documentclass{standalone}
\usepackage{tikz}

\input{data}

\begin{document}
\begin{tikzpicture}
  \draw (0,0) -- (\mylength,\mylength);
\end{tikzpicture}
\end{document}

Of course, in practice one will not add the \includegraphics until after the standalone has been run.