Switching to AMD would still likely be a big chunk of work (both for Apple optimising the OS, and for developers updating apps) if you wanted to maximise the benefits. I think too any people see AMD as an easy drop in replacement, but despite being x86 their chips are still going to be very different to Intel's. Unlike Windows which is designed to try and get on with all platforms as best it can, MacOS is a finely tuned OS which has only had to run on Intel chips for over a decade.
I don't think this is true. Of course they need to work on driver support, but even the Hackintosh community gets macOS running on AMD systems quite easily. Catalina is x86_64 only and that instruction set was developed by AMD, Intel has a (cross)license to use it as well. Apple started migrating some Intel-only features (like QuickSync) to the T2 chip. The T2 chip might do a better job at these tasks but the main benefit from this all is they are less dependent on Intel. Also from the past we know they've compiled every software project for both Intel and PowerPC for many years. Apple has grown immensely since then. I'm sure they've labs running macOS and other projects on many, many different architectures and systems.
Intel really has a huge problem. They are years behind AMD at this moment, Apple is dropping them within 1-2 years and other manufacturers are moving to AMD (and even ARM is gaining marketshare in the server business).