Hegel’s many references to Aristotle should help to clarify the obscure phrase that “Substance is also Subject”. In particular, Aristotle’s own somewhat obscure phrase about the identity of thought with the thing thought is relevant, as is his dialectical development of the different senses of ousia (“substance”) in the Metaphysics. As with Brandom’s Frege, a thought for Aristotle just is an inferentially articulable meaning, not a psychological event. What we care about in thought is shareable reasoning.
Thought in this sense is essentially self-standing, and unlike the mental-act sense not dependent in its determination on a “thinker” who optionally instantiates it. This gives a nice double meaning to the autonomy of reason. (What such thoughts do depend on is other such thoughts with which they are inferentially connected.) The primary locus of Aristotelian intellect is directly in shareable thoughts of this sort and their interconnection, rather than in a sentience that “has” them. Hegel adopts all of this.
Concepts in a unity of apperception are forms to be approached discursively, not mental representations or intentional acts. They are more like custom rules for material inference. The redoubling implied in apperception hints at the recursive structure of inferential articulation. The Absolute, or “the” Concept, just nominalizes such an inferential coherence of concepts.
Consistent with this general approach, I consider the direct locus of the subject-function to be in things like Brandomian commitments and Kantian syntheses. The subject-function is also indirectly attributable to “self-conscious individuals” by metonymy or inheritance, and to empirical persons by a further metonymy or inheritance. (See also Subject; Substance; Aristotelian Dialectic; Brandom and Kant; Rational/Talking Animal.)