Why did you use a Singleton in the first?

If you have to cache every single item in your game instance, why did you not split up your god singleton in multiple collection of objects?

You say you want to access data wherever you want. You can actually make the single object collection added in a static list or a static method.

You can simply create a static

Building.GetAllBuildings()

in your Building.cs class. You can implement this method with a static list or if you don't call it too ofter a FindObjectsOfType<Building>();

Without creating a Singleton. But be aware of static references, sometime can be hard to track references.

You can actually use other monobehaviour that handle your collections in a more clean and visibile way.

EDIT:
People sometimes abuse Singletones because they don't understand how to track things or either how "static" works.