I found Mitchell Hashimoto's My Approach to Building Large Technical Projects via Simon Willison's link post. Man, everything in this post is golden. I think I agree with every word. This is definitely how I approach building software. I especially love the focus on getting to something workable as soon as possible.
Building a demo also provides you with invaluable product feedback. You can quickly intuit whether something feels good, even if it isn't fully functional. These aren't "minimum viable products", because they really aren't viable, but they're good enough to provide an engineer some valuable self-reflection.
I love that phrase "self-reflection." We need more of that in software these days.
I also love the idea of building software for yourself. I've got some things I've been meaning to work on that I need to get back to. As I do, I'm going to get to a workable demo as quickly as I can. Maybe I'll have something to show here soon.