This exact thing happened to me with the update to Catalina. Had to send to fix, they were pricky arses over the ordeal. Sent back, it was never the same. Actually started seeing rainbow loaders.

The most damning permanent damage was that the wifi card never worked properly again and I had to connect an adapter, then a usb to ethernet adapter and use that. Which meant I had to sit in a specific disk at work if I wanted to use that computer.

To me, Catalina is the Vista of the mac world.