I like the adjacent forage pond idea. I have a pond that is little enough to throw my cast net across when half full. Twice now I have put in about 5 small adult bluegill in the spring and by the time the pond goes dry in August I have pulld out at least a thousand fingerlings. Remember you are adding bluegill to the pond at the same time thinning the bass so that the bluegill in the lake have a higher survival rate too. That should turn things around.
If you build a forage pond close enough you can pump water from the lake into it and keep it full.