It sounds like you have a good Ideal as to what is happening to this lake. Crappie (stunted and over populated) can cause problems in a lake. This lake is fairly large in size. (Will be harder to manage)
The threadfin shad are a good source of food for crappie, bass adult blueguill ect.. But the blueguill have most likely suffered with the compitition for food from the threadfin. "Fry compete for the same foods as threadfin". So depending on the fertility of the lake, proper balance can be difficult to get.
This can create an occasional Trophy Bass! if they can reach the size it takes to eat crappie as a food source. You will find that most bass will be either small short and fat from eating threadfin and can reach the 2 to 3 lb. mark fairly easy but that food source that carries them over to the next level could be missing. That is usually a good supply of bluegill in the 4 to 6 inch range.
You can never go wrong by creating plenty of cover for your baitfish in the 2 to 6 ft. range. Around areas that your blueguill spawn. ( We sank 300 christmas trees in a 45 acre lake to start with) You can never eat enough crappie! (We have caught 100+ in one sitting on one lake with the same problem) If you don't eat them at least Kill Them! The more you kill the bigger they will get. (We regularly catch 2+ pound crappie now.)
If this is a community lake, perhaps get others to join you in management efforts. Get with the local fisheries biologist and get their input. You may be able to poison the lake targeting the threadfin.
Knocking back the threadfins could knock back the crappie, which could rebound your blueguill, possibly getting the lake back in some sort of a managable state. It is possible, with poison to control certain things in a lake. (We have managed to get most of our 9 lakes in good shape with only hook, line, sweat and patience.) We are still working to keep it that way. Poison is a dangerous thing to a lake if not done right. If done right it can be a good thing.
If you have spotted bass in the lake it could be that the spots are also out of control competing with the large mouth and you should treat them just like you would the crappie in my opinion. (The more you kill the bigger they will get) Good Luck if you decide to try your hand at managing this lake. There is a ton of info on this site and if you can learn to read a lake like a book you will be way ahead of the game. Best advice I can give you is PATIENCE!!!!!!