A very, versy simple way to save and load games in Godot Engine

save_load.gd

extends Node

func save_game():
	var scene = PackedScene.new()
  
	scene.pack(self)
	ResourceSaver.save("user://save_01.tscn", scene)
	
func load_game():
	var scene = load("user://save_01.tscn")
	get_tree().change_scene_to(scene)

You don't want to put this on every node. Instead, you can turn it into a reusable node that you put around the root of your main game scene:

func save(branch, save_id):
    var scene = PackedScene.new()
    scene.pack(branch)
    ResourceSaver.save("user://save_%s.tscn" % save_id, scene)
	
func load(id):
    var scene = load("user://save_%s.tscn" % id)
    # change scene or return the loaded scene