Tenancy is the decision you'll live with longest
Every SaaS product makes an early call about how customer data is separated, usually without noticing. A table of users with a company name column works for the demo. Then a customer asks to invite their team, another wants two workspaces, and a third wants their data deleted completely. Suddenly the shortcut is in every query.
We work out the shape up front: organisations, memberships, roles and how data is scoped to each. It's a few days of thinking that saves months of migrations, and it makes things like per-team permissions and clean data exports straightforward later.