Shauna, congratulations on your upcoming graduation and on your job offer! Welcome to the forum.
I don't live in your country, so I have no first-hand experience with your future employer. I do, however, have experience with security clearances. Having a security clearance means that you can have NO personal secrets. If they find that you have a secret, you are deemed to be vulnerable to blackmail, and you lose your clearance. That usually means you lose your job.
It would be prudent to start planning how to come out to your future employer. The sooner you come out, the less vulnerable you will be to losing your clearance and the job.
Of course, being transgender in the military is problematic in your country right now, with no one really knowing where they will stand in the future. How much of that will trickle down to civilian DOD employees, I don't know, but it is something that you should be actively researching.
Good luck with it!