If they did an AMD CPU using the same TSMC node that Apple uses for Arm CPUs it wouldn't be that much less power efficient and have much great compatibility.

They would realistically gain the most efficiency by getting Nvidia to design a modern super power efficient GPU like what was used in the original switch and Nvidia Shield. AMD GPUs can be great for desktop gaming but in terms of power efficiency to performance ratio Nvidia is way ahead

An AMD CPU and Nvidia GPU might be a hard thing to actually negotiate however given that AMD is big in the GPU space as well. As far as I know most "APU" aren't really that special and just a combo of GPU and CPU

APUs have the GPU and CPU on the same package, or sometimes even the same die (with tiling). If there was to be an Nvidia GPU and AMD CPU type system, they would have to be separate packages.