World Anvil has a video about agile worldbuilding which I found useful for addressing the scoping problem I was thinking about yesterday. It's essentially a structured, ordered list of questions with a length limit on answers to get things started. The questions start from the broad strokes in, and you answer each one with just one short sentence. ("Short" in the sense that it's under 30 words, not explicitly stated but going by their examples.) I was surprised: One sentence each made a big difference in how detailed the world felt.
I didn't actually answer the questions fully in order, because I already had a foundational "magic rule" in mind, but I still found it helpful to go through each of the "Foundational" questions group. Answering each question in just one sentence, maybe two, made the process far more manageable. It also feels a lot better than having nothing, or only a vague idea, to start with.
I haven't finished the scene questions because I started late last night, but those seem useful as well.
I used this to set up a tabletop RPG world, which is their focus in the video, but I wonder if it works for writing as well. It seems like the questions would apply equally well. I expect my neuroses around writing fiction would be a challenge, but it's probably worth trying anyway.