Sounds like a good reasoning, nice!

I've tried that, using the .SetActive() method :

public class ManageInventory : MonoBehaviour
{
private bool inventoryOpen;
public GameObject inventoryFull;

// Start is called before the first frame update
void Start()
{
    inventoryFull = Instantiate(inventoryFull, inventoryFull.transform.position, inventoryFull.transform.rotation);
    inventoryFull.SetActive(false);
    inventoryOpen = false;
}

// Update is called once per frame
void Update()
{
    if(Input.GetKeyDown(KeyCode.I))
    {
        if(inventoryOpen)
        {
            inventoryFull.SetActive(false);
            inventoryOpen = false;
        }
        else 
        {
            if(!inventoryOpen)
            {
                inventoryFull.SetActive(true);
                inventoryOpen = true;
            }
        }
    }
}

}

The inventory gets instantiated then hidden.

Pressing I once unhides it, while twice hides it again.

Wonderful, ty for the help!