I like Overtons suggestions regarding alternate and/or additional forage species if you have an adequate plankton bloom. If it were me, and it's not, I would consider adding both tools for a couple of years. Both are prolific spawners. This would help the bluegill to rebound by taking pressure off them. I think that would be a lot better alternative than just adding bluegill. No bigger bluegill than 6 inches says that they don't get the opportunity to get large.
No matter what you do, you will have to continue monitoring and culling. More forage means more and larger bass and can turn into a Catch 22. However, the balancing of predator and prey is always critical and in a 15 acre lake can be time consuming and expensive. I envy you for having the challenge.
Tilapia die at 54 degree water temp and, I believe, threadfin at 42. However, as a tool, they will have done their job. You have to continuing doing yours.
Where is the lake in central Texas? That might matter regarding temps. You didn't mention the crappie and catfish. What is their situation?
