There is a massive debate about analog hardware (UAD, API, Neve) versus stock plugins. A great course remains agnostic.
The best mixing and mastering course will teach you the principles. mixing and mastering course
Skills are transferable; gear is not. If a course tells you that you need a $2,000 outboard compressor to get a good sound, run away. The pros get great sounds because they have great ears and a great system, not because they have a rack of expensive gear. There is a massive debate about analog hardware
Transitioning from "Mixing Engineer" to "Mastering Engineer" mindset. Skills are transferable; gear is not
Mastering is a dark art to many, but it is simply the final polish. A good course covers:
By the end, you will: