At the time I filed my I-140, the estimated waiting time for approval was 6 months as well (according to official processing dates). I got lucky I guess, also TSC was moving at lightning speeds back then - and it's still pretty fast I believe. So, you CAN sometimes get approved BEFORE processing dates catch up with your RD.
However, you are wise to choose to err on the side of caution. EB-1 EA and NIW are hard I-140s to get approved (I assume those are your categories since you mentioned "self-petitioning"). But the good news is, if by chance you were to get either approved before you have to leave the country, you could then immediately apply for I-485 (have your medicals ready!). And at that point, unless you continuously miss your FP appointments, or neglect to attach your medicals, or leave the country without AP, or have something criminal dug up in your name check - your I-485 approval should be a matter of time. And EB-1 EA and NIW cases are often waived interview.
At any rate, before you get on the plane to leave the U.S. and start the immigration journey from scratch, do invest in a consultation with a good immigration attorney to discuss all your options!
Good luck to you.