Applied at defense company for Unknown

I'm currently navigating a tricky situation regarding my job transition. Back in June, I interviewed for a position at a defense company and signed a written conditional offer, with a start date set for July 27. The offer hinges on passing three background checks and obtaining national and NATO security clearances. So far, I've successfully passed two of the checks and completed my onboarding by the end of June. However, the final NATO clearance is still pending, and I know that it can take several months due to its complexity. After spending the last two weeks on vacation to use up my remaining time off, I'm back at work and contemplating giving my two weeks' notice. But I'm feeling uncertain about whether this is the right move. I reached out to the security officer at the defense company for guidance, but she couldn't provide specific advice on timing. She did confirm that having the first two clearances is sufficient for me to start working. Despite feeling that I've made significant progress in the hiring process, I'm also anxious about the possibility of something going wrong and jeopardizing my new opportunity. I'm torn between wanting to move forward and the fear of the unknown. Would you feel comfortable putting in your notice under these circumstances?