You make good points. But then again, DS9 almost directly refutes the references in BoBW about 'the fleet' being the whole fleet. And just which fleet was that anyway? It could very easily have been just the home fleet - the tenth fleet for example. And from what DS9 indicates, a fleet is generally comprised of 100ish ships - so that might have only been half of the tenth fleet, that guards the core of Earth, and it might have been that to which Shelby referred, rather than the whole of the fleet.
And DS9 seems to directly imply that there were more than just six Galaxy
class ships built. Maybe even more than the alleged maximum 12, even if that was the original plan. And there seems to be a fair share of Nebula
class ships. Plus, Excelsior
class nacelles are relatively small, so if the coil material is the primary limiting factor of shipbuilding, lots of Excelsior
class ships might be advantageous over lots of larger ship types.