For at least the next 12 months, focus 100% on your full-time job.
Dealing with a lower GPA is something you should do when you're ready to apply. Again, resist the temptation to be preoccupied with stuff that is 2 steps ahead. If you do that, it's a guarantee that you won't make the most out of your current experience in your job.
The biggest challenge facing you now is building your professional career. That means full-time work experience, not business school, industry credentials, taking extension courses, etc.
If you're worried about your job prospects and promotions, then that's what you should be focused on - doing what you can to learn and achieve in the workplace. And if the job isn't offering you that after a while, then it's time to find another one.
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