I'm an engineer and talking from experience here.
1. keep the db schema simple. remember you are startup things change, so does the db schema so does your architecture.
2. One piece of advice from personal experience, Microservices usually don't work well for startups try building your solution as a modularized monolith.
3. always make decisions that are reversible (2 way door) and make sure coming back is also easy.
4. for any features you release have qualitative and quantitative metrics and more importantly guard rail metrics.
5. where ever you can use the an off the shelf solution like feature flags (launchdarkly), product analytics (amplitude) use them. These are solutions used by countless startups do not reinvent the wheel.
6. prioritize customer feedback, that is going to get you money and they are going to help make your product better. Instead of "failing fast" prioritize on "learning fast".
7. Be focussed on solving the problem don't romanticize the solution.