PlayerLoop挿入くん。シリアライズしてEditorからタイミング指定できる。

SerializablePlayerLoop.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.LowLevel;
using System.Linq;

static class PlayerLoopInjector
{
	static Dictionary<System.Type, System.Action> actions = new Dictionary<System.Type, System.Action>();

#if UNITY_EDITOR
	static PlayerLoopInjector()
	{
		UnityEditor.EditorApplication.playModeStateChanged += change => {
			if (change == UnityEditor.PlayModeStateChange.EnteredEditMode)
				actions.Clear();
		};
	}
#endif

	public static void AddAction(System.Type timing, System.Action action)
	{
		if (!actions.ContainsKey(timing))
		{
			var loops = PlayerLoop.GetCurrentPlayerLoop();
			loops.subSystemList = loops.subSystemList.Select(set =>
			{
				var index = set.subSystemList.Select(s => s.type).ToList().IndexOf(timing);
				if (index < 0) return set;
				set.subSystemList = set.subSystemList.Take(index + 1).Append(new PlayerLoopSystem()
				{
					type = typeof(SerializablePlayerLoop),
					updateDelegate = () => {
						if (actions.ContainsKey(timing))
							actions[timing]?.Invoke();
					}
				}).Concat(set.subSystemList.Skip(index + 1)).ToArray();
				return set;
			}).ToArray();
			PlayerLoop.SetPlayerLoop(loops);
			actions[timing] = null;
		}
		actions[timing] += action;
	}

	public static void RemoveAction(System.Type timing, System.Action action) {
		if (actions.ContainsKey(timing))
			actions[timing] -= action;
	}
}

[System.Serializable]
public class SerializablePlayerLoop
{
	[SerializeReference]
	object timing = new UnityEngine.PlayerLoop.Update.ScriptRunBehaviourUpdate();

	public event System.Action OnPlayerLoop
	{
		add => PlayerLoopInjector.AddAction(timing.GetType(), value);
		remove => PlayerLoopInjector.RemoveAction(timing.GetType(), value);
	}
}

#if UNITY_EDITOR
[UnityEditor.CustomPropertyDrawer(typeof(SerializablePlayerLoop))]
class SerializablePlayerLoopDrawer : UnityEditor.PropertyDrawer
{
	public override void OnGUI(Rect position, UnityEditor.SerializedProperty property, GUIContent label)
	{
		var prop = property.FindPropertyRelative("timing");
		var types = PlayerLoop.GetDefaultPlayerLoop().subSystemList.SelectMany(s =>s.subSystemList.Select(m => new {group = s.type, item = m.type })).Prepend(null).ToList();
		var currentIndex = types.Select(p => p?.item).ToList().IndexOf(prop.managedReferenceValue?.GetType());
		var newIndex = UnityEditor.EditorGUI.Popup(position, label.text, currentIndex, types.Select(t => t == null ? "<none>" : $"{t?.group?.Name }/{t?.item?.Name}").ToArray());
		if (newIndex != currentIndex)
			prop.managedReferenceValue = types[newIndex] == null ?
				null :
				System.Activator.CreateInstance(types[newIndex]?.item);
	}
}
#endif