Transmission intermittently slipping?

The only thing that can tie these problems together is the gauge control module, you can confirm the failure be doing a CAN scan, you can get a CAN scanner (Controller Area Network) from Amazon.

here is a video to show you how:

https://youtu.be/u-4syLc-ifQ

and

https://www.2carpros.com/articles/can-scan-controller-area-network-easy

Here are the instructions for replacing the gauge module which you can get a rebuilt unit by searching google or eBay. Check out the images (below). Please let us know what happens.

I checked the transmission fluid, and it was very low. I added fluid, and although there is still some transmission slipping, the problem is not nearly as bad as before. There are still odd electrical things going on intermittently. The last time I drove the car, it didn't seem to have a care in the world.
We connected a CAN scanner to the car today. This is the only thing the scan reported.

P0420
Catalyst system efficiency below threshold bank.

What steps should I take now?

With the transmission fluid low, it could have burned up the clutches or there is a change the input shaft speed sensor is off causing the slippage. Here is the location of the sensor so you can change it out. Also, can you recheck the fluid condition and smell if it is burnt? Let me know.

I took my car to a trusted transmission specialist on Friday. He said the transmission is fine, but my catalytic converter has become brittle with age, and the car just isn't getting the air it needs to function properly. My friend said that he can replace the converter for me. He found a very inexpensive one on Amazon. Is that a good idea?

Yes, this can cause the code to come on, but I don't think it will help with the other issues such as the transmission not coming out of park, clock display, vanity lights and radio speakers. I would say the gauge control module is going out. Here is the location I would tap on it to see if anything changes while the engine is running. You can get a preprogrammed unit by searching Google or Ebay. Check out the images (below).