For God is spirit.
Snowman, calling ‘by the Spirit’ a loophole is adorable. 
right.
God is spirit.
not
God is composed of spirit.
That would be contrary to Christian theology, particularly the simplicity of God – meaning there is nothing which is not God by which God is. This excludes composition as well as contingency in general.
For other spiritual things, I am only suggesting they are not composed of anything, and not to say there is nothing contingent about them. This includes other examples like angels, which are also spirit but are created and therefore contingent. Many wonder… if this is possible then why bother at all with the composed existence of physical things? It because this would limit things only to those which are no more than what God made them to be. The idea is to provide a way for things to participate in their own creation… to grow, to learn, to make their own choices. And when made in ones own image, this is the idea of children.
And… even in this, the basic idea of existentialism is there… not as an absolute but only about our particular nature as physical beings who choose what to do with the existence provided them and in that choice is their essence.
God is spirit. There is also a possibility that the spiritual world with the created spiritual entities was before the life on Earth started.
At what point God created the unseen world is a mystery, we do not seem to have revelation about it. Nor scientifically valid data of anything in the spiritual world. I do not believe that Genesis 1 is a historical description of the actual creation of this universe but it seems that in the story, the spiritual entities were present when God created the material life.
And, of course, theologically, metaphysically, God is a substance.
Yes. That is what I believe. Before the creation of the physical universe, even.
I think the creation of angels was the first logical step in God’s creation of others for a relationship – the simple obvious easy way. It was seeking something more satisfying that led to the vastly more complicated approach of creating the physical universe to enable the self-organizing process of physical life.
That is the much older philosophical use of that word “substance” more as a translation of the word “ousia” which really means being, essence, nature, or reality. Looking up the word "substance in most English dictionaries often will not give this meaning at all and what you find there certainly is not what is meant if the word “substance” when used for God. Personally, I think it is a flaw in ancient Greek thought before modern science which practically imposed artifacts of language on reality itself. Only with modern atomic science did we have an alternative to start with a material reality apart from language and thus could remove this bias in ancient Greek thought.
I don’t think of God as just another (even if ‘oldest’) member of the created universe. God is not part of any lineage of creation, at least not apart from incarnation. But even so, you may be right about other spiritual things generally. Far be it from me to be dogmatic about spiritual timelines.