Not sure about this kind of case, but for more garden variety crimes it is common for the first charges to be slam-dunk, with tons of evidence and a clear path to conviction.

Then, during the pre-trial phase, a superseding indictment can be added as more evidence is gathered and a stronger case is put together.

This happened with Paul Manafort, who was first indicted for “conspiracy against the United States”, but then the superseding indictment included all sorts of tax and bank fraud.