I think there is a lot of value in what you're offering (the ability for product owners/requirement definers/etc. to define what is needed on a screen by screen basis). But that wasn't at all clear to me, as a frontend dev, on seeing your product page. I thought it was some sort of UI mockup tool. But that's not really the point at all, is it? It's more like a really precise checklist with built-in flows and prototyping.
What caught me was -- counter-intuitively, perhaps -- how polished the cards were. They look like UI mockups even though you say they're not really mockups. I thought they were finished modals that I was going to implement. No, they're really more like (or even less than, actually) something like Balsamiq (whose sketched look makes it obvious that they aren't designs, just ideations). In contrast, the polish your app cards had suggested that they were nearly finished UI mockups and actually worked AGAINST the idea that "hey, nevermind the layout or components, we just need screen X with this and that, which leads to screen Y with this and that". In other words, as a dev, I was too distracted by the polished UIs and how to implement them to realize that it was actually just a way to document requirements and flows (which would indeed be very useful vs something like a Jira ticket with a bunch of bullets).
I don't mean this as a criticism, but more like "oh, now that I get what they're going for... I can see the value". I hope that's a helpful datapoint...
As for the high fidelity, UI Drafter doesn't use a hand-drawn style because it would make it look like a wireframing tool, which would be worse than your initial impression.