Thanks! Caveat: I don't know Substack that well yet. But my intuition is that, yes, I would personally use Substack for it. It feels ideal for what you're talking about.

I would also consider cross-posting your serial fiction onto Medium (but keeping your Substack as the primary) with a REL Canonical backlink to your substack story. I would try to find a fiction pub on Medium that will allow this.