Might still revise this, but my current view on the Al-A'yan Al-Thabitha is that its correspondent in Proclean metaphysics is Intelligible Being: Existent unity/unities that are not manifest but still "known" by the God.
Unity for Schuon is simply “Self”
“God” for him requires something to be God of, an implicit dualism.
This idea of Self beyond even “God” as a name has a long history in Sufi metaphysics.