Determine the PlotRange of a Graphics object programmatically

You can use my GraphicsInformation function to obtain this information. Install with:

PacletInstall[
    "GraphicsInformation",
    "Site" -> "http://raw.githubusercontent.com/carlwoll/GraphicsInformation/master"
];

Then, load it:

<<GraphicsInformation`

For your example:

GraphicsInformation[expression]
"PlotRange" /. %

{"ImagePadding" -> {{0.5, 18.}, {11.4785, 24.}}, "ImageSize" -> {400., 261.553}, "PlotRangeSize" -> {381.5, 226.074}, "ImagePaddingSize" -> {18.5, 35.4785}, "PlotRange" -> {{-44.5312, 307.031}, {-4.16667, 204.167}}}

{{-44.5312, 307.031}, {-4.16667, 204.167}}

Note that GraphicsInformation is more reliable than PlotRange. For instance, compare:

PlotRange[Graphics[{}, Axes->True]]
"PlotRange" /. GraphicsInformation[Graphics[{}, Axes->True]]

{{0., 1.}, {0., 1.}}

{{-1.04167, 1.04167}, {-1.04, 1.04}}