First is don't play thresh in bronze because the ADC probably won't click the lantern. That's a lot of power in his kit and if your ADCs aren't making use of it, it's going to waste. Now keep practicing him in normals as he is one of the better supports to know and you need a lot of games on him to fully use him. If you are that good with his hooks (and I really hate to say this) but have you tried blitz?
Second, Soraka is fine and you might also want to pick up Janna and Sona in case Soraka is banned or picked.
Third, look into someone like Annie or Zyra. These are carry supports who can really help the team out and turn into off-mages.
Finally, remember you can climb out of bronze as a support main. If your ADC isn't doing well by mid-game, then ditch him and support whoever is hard-carrying your team. Remember to ward because you are probably going to be the only one to do it, but never ward alone. Use pings and map awareness to help your team. After you finish your eyestone, boots, and any item that might be core to your support, look into damage items as sometimes more damage on anyone (even AP soraka) is good enough for bronze.