I agree with all of the above. If you have plentiful small bass you certainly don't need more.
I believe I would pay attention to the forage base of bluegills and try to grow them. The bass will follow.
If the fishing for large bass has declined, it is because they aren't getting the proper amount of forage(food) that they need. The bass need forage that is between 1/4 to 1/3 their body size and the old rule of thumb is that it takes 10 pounds of groceries for a bass to gain 1 pound. That 1/10 is often debated here. You just might have a situation where the small bass are over eating the bluegills before they get a chance to grow large enough to make a proper meal for the larger predators.
Think about this. The best cattle ranch manager is a grass farmer. The cow is the predator and the grass is the prey(forage). If the cows over graze the grass, the ground bakes and has a problem replenishing itself. However, if the cows are rotated around, the forage base has a chance to replenish. Culling of the herd is also vital. If the cow has to walk too far in search of food, it loses weight and fails to thrive. Thus, both the predator and forage stay healthy when both predator and prey are balanced. It's the same with rabbits and coyotes.
Like others here, I would hire a professional with a good reputation for lake management and not fish selling. If that is a dead issue, I think you are hosed and need to tell them to find someone else to waste their money. From what you are saying, I think restocking without not knowing the current situation is a good way to blow $20,000.
Actually, it sounds like, to me, that more fish is not what you need AT THIS TIME.
