This sounds like a good approach, and I'm glad it has helped you make progress.

I've never completed a long-form writing project yet, but all of my WIPs are in Scrivener. I like how I can plot out scenes in a few bullet points in the corkboard view, then switch to the writing view for that particular note and write out the scene in full, before joining them all up later. It sounds similar to what you're doing, but your method is even more free – write what you want, even if it isn't a "planned" scene yet.

Just by getting started, I'm sure those 500 words will develop into more quite naturally!