You ask about EGT v. ITT in a turbofan engine and then give a good explanation of leaning a piston engine. Totally different operations. You don't lean a jet engine. There is no "red lever." The fuel control unit will figure out how much fuel to send to the engine based on thrust lever position and outside conditions. On a turbine engine both EGT and ITT have limitations you do not want to exceed. Maintenance can track the numbers to monitor engine health but for pilots it's just a number.