Threads? Or just Coroutines?

HI. I'm having a bit of a problem with a code that takes half a second to process. Unity tries to complete the process in 1 frame before continuing, causing a small lag spike that players shouldn't need to see, so I found that I could use Coroutines or Threads to fix that.

What I need is to save stats and inventory (I'm using Easy Save and Inventory Pro), I tried using coroutines but I would have to split the code that saves inventories in between "yield return new WaitForSeconds();" and I can't do that, so I chose to use Threads.

This is what I have:

 // OUTSIDE METHODS, AT START OF CLASS
     Thread oThread = new Thread(RestAtWell.savingGame);
 
 // THIS GETS DATA TO SAVE, BECAUSE THREADS CAN'T REFERENCE FROM MAIN THREAD
     public void GetSaveData()
     {
         playerStats = GetComponent<ES2_SaveStats>();
         var serializers = FindObjectsOfType<EasySave2CollectionSaverLoader>();
         foreach (var s in serializers)
         {
             collections.Add(s);
         }
     }
 
 // WHEN THE OBJECT IS USED, START SAVE
     bool ITriggerCallbacks.OnTriggerUsed(Player player)
     {
         GetSaveData();
         oThread.Start();
         return false;
     }
 
 // THIS IS THE THREAD
     static void savingGame()
     {
         // save Stats
         playerStats.SaveStats();
 
         // save collections
         foreach (EasySave2CollectionSaverLoader s in collections)
         {
             Debug.Log("Saving Collection");
             s.Save();
         }
     }

So basically, when I trigger this, there's no Debug showing, there are no saving but the threads seems to be started, because if I trigger again, I get an error saying the thread has already started. This is a problem because I want this code to save the game every time the player interacts with the checkpoint.

I know I can make this work because placing the: "Thread oThread = new Thread(RestAtWell.savingGame);" right before "oThread.Start();" makes it work, it saves the data without lagging, but the next time I trigger it, the data is saved twice, then thrice. I think that the code starts duplicates of the thread multiple times or something like that.

So how could I make this thread do its thing, stop, and then be able to start again when needed? I haven't found much info about threading, I know unity doesn't like this. If you know a better way for me to implement this, I'm all ears.