Center the scene on the screen when not main scene in Godot

center-scene.gd

# Add to main autoload file

func _ready():
	# Check the last child node of the root
	var scene_node = get_node("/root").get_children()[-1]
	if scene_node.filename != ProjectSettings.get_setting("application/run/main_scene"):
		var property = "position"
		if scene_node is Control:
			property = "rect_" + property
		set(property, get_viewport().size / 2)

You can add and not "pin" in scene_node for example to the if statement where var pin is added to scene scripts where you don't want the centering to take place.