Mid September should be good. Get a map of the lake, find and work drop offs, ledges.
Work cover such as docks, piers and logs, particularly if they are adjacent to deeper water.
Keep the downriggers at home and use that space to bring as many rods already rigged as possible. Have a rod with a spinner bait, another with a tube jig, texas rigged plastic worm, shallow and deep diving crank baits and/or jerk baits and have some fun. I typically fish colours on the natural side in the fall, but everyone has their favourites. Worms/jigs in browns, purples, and black.
The bass will be active before the cold hits. I would give you my 'secret spots' but I'd have to kill you.
There are many quality places to fish on that lake. Don't be afraid to keep moving from weedy areas to rocky areas and everything in between, but again, look for depth changes and structure.
Don't forget to troll around the ledges for trout as well, there are nice ones in there that could be getting active again at that time of the year.
Best of luck.