MenuItem.cs

using System;
using System.IO;
using Unity.RuntimeSceneSerialization;
using UnityEditor;
using UnityEngine;
using UnityEngine.SceneManagement;

public static class MenuItem
{
    [UnityEditor.MenuItem("Sandbox/CreateAssetPack")]
    public static void CreateAssetPack()
    {
        if (Application.isPlaying)
        {
            Debug.LogError($"Application.isPlaying");
            return;
        }

        Debug.Log("CreateAssetPack");

        var activeScene = SceneManager.GetActiveScene();
        if (!activeScene.IsValid()) return;

        var assetPackPath = "Assets/Scenes/Game.asset";
        var assetPack = AssetDatabase.LoadAssetAtPath<AssetPack>(assetPackPath);
        var created = false;
        if (assetPack == null)
        {
            created = true;
            assetPack = ScriptableObject.CreateInstance<AssetPack>();
        }
        else
        {
            assetPack.Clear();
        }

        var renderSettings = SerializedRenderSettings.CreateFromActiveScene();
        var sceneAsset = AssetDatabase.LoadAssetAtPath<SceneAsset>(activeScene.path);
        if (sceneAsset != null) assetPack.SceneAsset = sceneAsset;

        SceneSerialization.SerializeScene(activeScene, renderSettings, assetPack);

        if (created)
        {
            AssetDatabase.CreateAsset(assetPack, assetPackPath);
        }
        else
        {
            EditorUtility.SetDirty(assetPack);
        }

        AssetDatabase.SaveAssets();
        AssetDatabase.Refresh();

        Debug.Log($"Complete! {assetPack.AssetCount}");
    }
}

Sandbox.cs

using System;
using System.IO;
using System.Text;
using Cysharp.Threading.Tasks;
using Unity.RuntimeSceneSerialization;
using UnityEditor;
using UnityEngine;
using UnityEngine.SceneManagement;
using UnityEngine.UI;

public class Sandbox : MonoBehaviour
{
    private Scene _bootScene;
    private Scene _gameScene;

    public void Start()
    {
        Debug.Log("Sandbox.Start");
        _bootScene = SceneManager.GetActiveScene();
        _gameScene = SceneManager.LoadScene("Game", new LoadSceneParameters(LoadSceneMode.Additive));
    }

    public void Update()
    {
        void SaveOrLoad(int index)
        {
            if (Input.GetKey(KeyCode.LeftShift) || Input.GetKey(KeyCode.RightShift)) Save(index).Forget();
            else Load(index).Forget();
        }

        if (Input.GetKeyDown(KeyCode.Alpha1) || Input.GetKeyDown(KeyCode.Keypad1)) SaveOrLoad(1);
        if (Input.GetKeyDown(KeyCode.Alpha2) || Input.GetKeyDown(KeyCode.Keypad2)) SaveOrLoad(2);
        if (Input.GetKeyDown(KeyCode.Alpha3) || Input.GetKeyDown(KeyCode.Keypad3)) SaveOrLoad(3);
        if (Input.GetKeyDown(KeyCode.Alpha4) || Input.GetKeyDown(KeyCode.Keypad4)) SaveOrLoad(4);
    }

    public void OnClickSave(int index) => Save(index).Forget();
    public void OnClickLoad(int index) => Load(index).Forget();

    private static string GetPath(int index) => $"Saved/{index}.json";

    private static AssetPack GetAssetPack()
    {
        var assetPack = AssetDatabase.LoadAssetAtPath<AssetPack>("Assets/Scenes/Game.asset");
        if (assetPack == null) throw new Exception("assetPack == null");
        return assetPack;
    }

    private async UniTaskVoid Save(int index)
    {
        Debug.Log($"Save{index}");
        var serializedString = SceneSerialization.SerializeScene(_gameScene, new SerializedRenderSettings(), GetAssetPack());
        await File.WriteAllTextAsync(GetPath(index), serializedString, Encoding.UTF8);
        Debug.Log($"Complete!");
    }

    private async UniTaskVoid Load(int index)
    {
        Debug.Log($"UnloadScene");
        await SceneManager.UnloadSceneAsync(_gameScene);
        await SceneManager.LoadSceneAsync("Load", new LoadSceneParameters(LoadSceneMode.Additive));
        _gameScene = SceneManager.GetSceneAt(1);
        SceneManager.SetActiveScene(_gameScene);

        Debug.Log($"Load{index}");
        var serializedString = await File.ReadAllTextAsync(GetPath(index), Encoding.UTF8);
        SceneSerialization.ImportScene(serializedString, GetAssetPack());
        Debug.Log($"Complete!");

        SceneManager.SetActiveScene(_bootScene);
    }
}