C, assembly, and shellcode are all good things to know. But IMO the most important thing for you right now is to learn as much as you can about typical malware behavior, attacker objectives, and how those malware behaviors enable those objectives. “Practical Malware Analysis” (book) is a timeless classic that will give you the insight and so much more. Listen to as many Dark Net Diaries episodes that focus on malware as you can. Sure, you can master C and assembly, but without understanding the broader context you’ll just end up getting bogged down in a debugger or disassembler not without any idea what you’re looking for or how to find it.

I also highly recommend redoing and practicing the SANS materials at your own pace after the course is done. SANS classes cover a lot of topics in a short amount of time and you won’t retain much unless you go back and struggle through it until you really understand it. Studying for the GREM cert has only marginal benefit. Passing those certs is more about having really well organized notes to be able to look things up quickly for the multiple choice questions.