Incorrect gear ratio codes in an automatic often means the transmission is slipping.

It's possible the turbine speed sensor is failing, but you would probably get a separate code for that.

The adage about not changing the fluid on a neglected transmission applies to traditional automatics that use clutches to produce the desired gear ratio. If the clutches wear out, the fluid with suspended clutch particles helps create a little more friction. Change that fluid, and now it's even worse. Your CVT uses a chain, so there is no clutch material involved (there are still clutches, but they don't work the same way, and won't cause the same issues). Given that, I'd say it's safe to try a fluid change just in case. As far as replacing the transmission, new ones are absurdly expensive, and a rebuild is risky because it's still a somewhat new technology, so not all shops have technicians with the right skill set and experience. If you can find a transmission shop in your area with experience in chain based CVTs, that's probably your best option. Otherwise, get a used transmission out of a wrecked lower mileage car, and hope for the best.