Hi, I was in a very similar spot to you about 4 months ago.

Some points I am not quite sure I understand:

(1) Why would you have to negotiate your salary if you already got the job? (and said pay and benefits are great) .Are you planning on doing freelance/ part-time and negotiating salary in your current company or outside of it ? (This does makes a different, it is generally considered "easier" to get a big pay raise transferring from one company to another , rather than staying in the same company. Not always the case, but as a general rule of thumb)

(2) Are you planning on doing part-time/ freelance on top or instead of your current position? (if it is on top- then for the sake of managing education as well as work, I would strongly advice against it, as managing a degree and one job is already quite a challenge, and you don't want to be overworking yourself)

(3) The advice I needed for managing work and education: learn how to say no to your manager. Not too regularly of course, but generally if your boss doesn't know your schedule at university , and is seeing that you are doing good, he might assign you, or encourage you to take more responsibility. It is good if you are on your break, and feeling up to it, it is not good if you have a really large assignment at school and the oonly reason you are capable of doing you job is