A simple UdonSharp script for globally synced slider. It works for late joiners, but I think its gameobject has to be active when Scene starts.

GlobalSlider.cs

using UdonSharp;
using UnityEngine;
using UnityEngine.UI;
using VRC.SDKBase;

namespace VRCTools
{
    [UdonBehaviourSyncMode(BehaviourSyncMode.Manual)]
    public class GlobalSlider : UdonSharpBehaviour
    {
        [SerializeField] Slider slider;
        // 1. Add SerializeField for objects that you want to change with slider
        // [SerializeField] Light light;

        [UdonSynced, FieldChangeCallback(nameof(SyncedValue))]
        float syncedValue;
        public float SyncedValue
        {
            get => syncedValue;
            set
            {
                deserializing = true;
                if (!Networking.IsOwner(gameObject))
                {
                    slider.value = value;
                }
                syncedValue = value;

                // 2. Use slider's value here
                // light.intensity = value;

                deserializing = false;
            }
        }
        bool deserializing;

        public void OnValueChanged()
        {
            if (deserializing) return;
            Networking.SetOwner(Networking.LocalPlayer, gameObject);
            SyncedValue = slider.value;
            RequestSerialization();
        }
    }
}