GS350

Hello Scotty. I hope you are doing well. I have a 2007 Lexus GS350 RWD Automatic 6 speed Transmission with 112k miles. Recently I noticed that the transmission jerks a little shifting from 1st to 2nd gear. Feels like the gear is catching late with a jolt. Especially happens when accelerating hard. Also I can smell something like burning rubber coming when driving hard. There is no vibration or shuddering or whining noise. The transmission doesn't slip as well. Car drives perfectly fine and the power and acceleration is normal. I have never touched the transmission fluid. I bought the car at 82k miles and now it has 112k miles. I took it to a transmission shop and they checked it for free. First they said there was nothing wrong then I told them to check again and they said the torque convertor failed and burnt the overdrive clutches and the fluid is burnt and black and gello type like mud. They said I'll need to rebuild or put a new or used tranny. They said they'll charge 3400 to rebuild and 5k dollars for a used tranny. I'm not sure if I should trust them since I did some research and it doesn't seem that my car has the symptoms of a failed torque convertor. You help will be highly appreciated.