Don’t get into tearing apart the dashboard without another opinion. At the moment this shop is just chasing the problem but not making any real headway. Doubtful the dash wiring is at fault, the transmission shop is a good suggestion. It sounds like the lock up feature of the torque converter is a part of the problem and the computer control unit for the transmission may be a part of the problem. When most transmissions have problems they trip the check engine light. The codes for the motor that he’s reading my have little to nothing to do with the transmission issues. Ask the shop you are using for a referral to a good transmission shop that knows Toyota brand cars. It they don’t have a referral, you’ll have to do the research yourself. As a last resort you may need to take this to a Toyota dealer and hope they have a good transmission tech.