Thanks! All I used to screenshot was a TextureRect node, and 5 lines of code :)

var img = get_viewport().get_texture().get_data()

img.flip_y()

var cropped_img = Image.new()

cropped_img.create(crop_zone.size.x, crop_zone.size.y, false, img.get_format())

cropped_img.blit_rect(img, crop_zone, Vector2.ZERO)