Start with one system you can understand
For many new web products, a single application with a relational database is a strong starting point. Keep features grouped by business responsibility and make the important flows easy to trace.
This makes it easier to ship, debug, and change the product while its real requirements are still emerging.
Be deliberate with your data
Model the concepts users care about, define ownership of each record, and keep changes to data predictable. Add database constraints for rules that must always hold. Plan migrations so the data can evolve safely.
A clear data model is more valuable than choosing infrastructure for a scale you do not yet have.
Production quality is a habit
Set up error reporting, backups, access control, and a simple deployment path early. Test the flows that would hurt most if they broke. Document how to recover from common failures.
The goal is not a perfect system. It is one the team can operate confidently and improve as the product grows.