I think you don't leave your job. Because may your freelancing is not guaranteed. But your job is secure. My suggestion is do part time and be happy with your job.
Do you enjoy your job? If you hate it, try to get laid off or fired so you can get unemployment while you work on your freelancing for another six months with pay. See how it turns out and then you can seek another job or wait until the unemployment runs out.
I think it would be better to be more professional on work and try to get hired by big companies at where you can earn a good money from the that company. I hope these would be better one for you.
if you are able to handle both works without any problem then there is no need to quit your job. After all by doing both works at the same time you are earning a lot.