Besides God no substance can be conceived
Formal Statement
Besides God, no substance can be granted or conceived. God possesses every attribute (Def.6) and necessarily exists (P11). Any other supposed substance would have to be explained by some attribute — but every attribute already belongs to God. Two substances with the same attribute are impossible (P5). Therefore God is the only substance.
In Plain Language
This is the moment monism locks into place. God has already claimed every attribute (that is what "absolutely infinite" means). If you tried to posit a second substance, it would need at least one attribute — but whatever attribute you pick, God already has it, and P5 says two substances cannot share an attribute. So no second substance is even conceivable. The universe does not contain God plus other substances; the universe contains God, period. Everything else is a mode.
Why This Follows
God is defined as having infinite (i.e., all) attributes (gs-04) and necessarily exists (gs-12). Any second substance would require at least one attribute, but P5 (gs-07) forbids two substances from sharing an attribute. Since God has them all, no attribute is left for a rival substance. Monism follows.
There is exactly one substance in nature: God.
Connected Concepts
If there can be only one substance, does it still make sense to call it "God"? What work is that name doing?