Well, you can keep throwing parts (and fluids/filters) at it, and you may decide to add a speed sensor if it is cheap enough, but that is generally an expensive repair approach.

If you didn't have the code and failsafe mode to start with, just the slipping, then there are two possibilities.

One is that you used the wrong fluid, and made an existing (slipping) problem worse. It was failing, and you accelerated it.

The second is that the fluid change masked the problem for a short while, until it got worse, as it would have done anyway.

Both cases call for opening up the transmission, unless you know that you had the speed sensor fault both before and after the slipping and fluid change. But if that was true, why didn't you have the failsafe mode prior to the fluid change?

Remember that the code likely doesn't say the speed sensor is bad. It says that the reading is implausable, ie, this combination of inputs shouldn't produce this output. That root problem can be mechanical, hydraulic, or electronic in nature. Deciding that it is just electronic seems like a faint hope.

I am going to top it off again with Pentosin and check the wiring harness. Should it snap back into place or just basically screw on?