At my large corp, we try to figure out and align stakeholders on the target tech architecture we want, and then sort the org structure out second as our way of

At my large corp, we try to figure out and align stakeholders on the target tech architecture we want, and then sort the org structure out second as our way of mitigating the distortions that come with Conway's law. More towards your question, having also been in a mildly successful startup, I think there is a certain amount of layering in your architecture that you can setup on day one (frontend vs backend), but so much success rests on execution and product fit and such that it's not worth wasting too many cycles overoptimizing the architecture if you aren't already opinionated on it. Go with the architecture that you can iterate rapidly with. While I haven't been a solo dev, I'd ask myself the question "what work will I offload once I can afford to, and what architecture do I need to create so that next 1-3 persons able to work without a ton of my time?" Iterate from there.