Is it possible to save _draw from viewport?

Godot Version

Godot 4.2.2 stable

Question

I’m using a subViewport to draw arcs and polylines, and then trying to screenshot the result. I seem to be able to screenshot the viewport, but none of what I made using the _draw function shows up.
I’ve been using the commands:

await RenderingServer.frame_post_draw
$Viewport.get_texture().get_image().save_png(res://images/screenshot.png)

as shown in the tutorial to at least see the results of my screenshots. And what I get is just the grey background of the subViewport. I can get the full screen on the main viewport if I want, just minus the drawn features.

Is it that these cannot be captured or is there a different method I should be using?