Why do my images look pixelated in Develop Module? I have just upgraded to the new Lightroom Classic

That doesn't really solve anything. This is a problem in your GPU/driver. Without GPU support many functions will be disabled.

Since this is a Mac you can't update the driver, it's all integrated into MacOS. But instead of disabling the GPU altogether, you can try to uncheck levels of functionality:

Or perhaps roll back to a previous LrC version (that maybe had slightly simpler/different GPU code that didn't trigger this).