What should a soft launch actually test?

Not whether invited guests enjoyed a free meal — whether the operation holds under designed stress. Capacity should step up night by night, the full menu should be ordered deliberately, ticket times and error rates should be recorded, and the failure points fixed between nights. A soft launch without measurement is a rehearsal without notes; the point is to find the breaking points while the audience is forgiving.

From The 45-Day Pre-Opening Discipline: What the Last Six Weeks Decide — the full reading, with the method behind this answer.