Match the shape to the problem. A platform partner is right when the answer really is that platform. An agency brings hands at volume. A solo operator is right when the problem is odd-shaped and crosses systems, because you get the person who scoped it doing the work.
A free or cheap diagnosis, a fixed scope with a named outcome, your tools and your accounts, and an exit that leaves everything running. Wariness at any of the four is not cynicism; it is pattern recognition.
Not very. You need to know your own process cold, and you need to be willing to say when a draft is wrong. The consultant's job is to make the technical part invisible.