FPSAdjuster.cs

using System.Threading;
using UnityEngine;

public class FPSAdjuster : MonoBehaviour
{
    [SerializeField, Min(1)]
    private int fps;

    [SerializeField]
    private bool useThreadSleep = false;

    private int interval => 1000 / fps;
    private int defaultFps;

    float prevFrameTime;
    
    void Start()
    {
        prevFrameTime = Time.realtimeSinceStartup;
        defaultFps = Application.targetFrameRate;
    }
    
    void Update()
    {
        if (!useThreadSleep)
        {
            Application.targetFrameRate = fps;
        }
        else
        {
            Application.targetFrameRate = defaultFps;
            float frameTime = Time.realtimeSinceStartup;
            float delta = frameTime - prevFrameTime;

            int requiredInterval = Mathf.RoundToInt(interval - delta * 1000);

            if (requiredInterval > 0)
            {
                Thread.Sleep(requiredInterval);
            }

            prevFrameTime = Time.realtimeSinceStartup;
        }
    }
}