If you have the extra money that you say you do by being able to pay $750 a month, why not put it to better use.
Your RC390 is just fine for a first bike and your insurance is already expensive by the general standards.
Thake that $200 and put in into savings of some sort. Or put it aside and take some riding courses as that will only improve your skills and make you ready for a 600cc+.
We get it, you are young and excited and sometimes don't necessarily make the best decisions with the most logical sense (that is not an insult by any means). Some of as have been there and done that.
If you come you here asking for advise, you must be able to take it and "think" about it, instead of just insisting why a larger bike is the right choice for you. We are all giving you sound advice based on experience and logic.
At the end of the day you are free to make which ever decision your heart desires.