You won't get 12 FPS it would be more around 40 - 50 FPS on Fortnite and you can look for deals on more high-end GPU's but without deals, I feel like that's the best option.
I don't think you can disable fans on any GPU. I may be wrong but I'm using the logic of "why would our customers want to disable a fan on the GPU". Best bet is trying to get the fan replaced.