UnityWaitToStartBehaviour.cs

// The magical Start() method can also be treated as a coroutine if it returns
// IEnumerator.  This lets a component class behave linearly, or at least start
// up with linear logic such as waiting for the right conditions to begin the
// Update() loop.

public class WaitToStartBehaviour: MonoBehaviour
{
    // all the usual Singleton stuff (optional)

    public VitalThing vital = null;
    public List<ImportantThing> importantThings = new();

    public bool IsEverythingReady() =>
        vital != null && importantThings.Count > 0;

    IEnumerator Start()
    {
        while (!IsEverythingReady())
            yield return null;
        DoStartupStuffThatNeedsEverythingReady();
    }

    DoStartupStuffThatNeedsEverythingReady()
    {
        //...
    }

    // all the other things this class does
}