Beamer: Uncover magnification using tikz spy library

Question

Using the spy library, I would like to first highlight (in this case with an rectangle) the area which gets magnified and the show the magnification on the next slide. I am doing this manually in the MWE below, but prefer an automatic solution.

\documentclass[10pt,aspectratio=169]{beamer}

\usepackage{tikz}
\usetikzlibrary{spy}

\begin{document}
\begin{frame}
\begin{tikzpicture}[%
   spy using outlines={rectangle, magnification=4, size=2cm, connect spies},
]
    \fill [blue!20] (0,0) rectangle (2,3);
    % I want to avoid to manually draw this rectangle:
    \draw (-2.5mm,-2.5mm) rectangle (2.5mm, 2.5mm); 
    \pause
    \spy on (0,0) in node at (-2,1);

\end{tikzpicture}
\end{frame}
\end{document}

The result should look like this:

Answer 1

Big thanks to @samcarter for explaining the question to me! Using some tricks from the aobs package.

\documentclass[10pt,aspectratio=169]{beamer}

\usepackage{tikz}
\usetikzlibrary{spy}
\tikzset{
    invisible/.style={opacity=0,text opacity=0},
    visible on/.style={alt={#1{}{invisible}}},
    connect on/.style={alt={#1{connect spies}{}}},
    alt/.code args={<#1>#2#3}{%
      \alt<#1>{\pgfkeysalso{#2}}{\pgfkeysalso{#3}} 
    },
}

\begin{document}
\begin{frame}
\begin{tikzpicture}[%
   spy using outlines={rectangle, magnification=4, size=2cm},
]
    \fill [blue!20] (0,0) rectangle (2,3);
    \spy[connect on=<2->] on (0,0) in node[visible on=<2->] at (-2,1);

\end{tikzpicture}
\end{frame}
\end{document}