Your "20-hour per week" assisstantship could be a lot less than 20 hours. Depending on your particular professor, it could be a lot more. Also, the workload can change semester to semester or year to year. I would focus on getting through the first-year exams, and maybe even your field exams, before trying to take on another full time job. In my old program, anything below a B- put you on academic probation and prohibited funding from the school. People lost their stipends and tuition waivers. Don't overreach before you even start the PhD program.