Most new practices die waiting to be "ready." The ones that live find a paying client before the infrastructure exists to serve a large one.
An anchor client is not your biggest prospect. It is the smallest one whose expectations are real enough to force the offering into shape — early, while the cost of being wrong is still low.
Choose the first client for the discipline it imposes, not the revenue it brings.
The first mandate does three things at once: it names the offering, it sets the standard, and it gives the rest of the organisation a reason to take the initiative seriously. Skip it, and you are building in a vacuum that no amount of internal conviction can fill.