After thinking about it, I don't think an annuity is the right choice for me. I have a son when I die, I'd like to have something substantial to leave to him. I think when the time comes I'll more than likely go with a fund like the Vanguard Managed Payout Fund: https://personal.vanguard.com/us/funds/vanguard/ManagedPayoutList.
I am definitely not a fan of annuities. But the right kind, from the right company can be ok in some situations. In yours, a SPIA might be worth considering. I would talk with Vanguard since they sell annuities as well but at much lower fees than others. The desire to leave a lump sum can be arranged as well.
Riddle me this. If one takes their pension (very annuity-esque), and turns around, runs their money thru the contraption known as the stock market, and then turns around and buys another annuity, what is gained? Personally, $1500 is plenty to cover my base expenses, and I'm almost certain I make significantly more than an e7. Of course, this assumes one has a paid off house, living below their current means, etc. File me under the camp of a person with a military pension probably never needing an SPIA.
To the OP, don't count your chickens before your eggs have hatched. Each year you go, the more likely you are to be forced out. Being enlisted, you probably offer nothing that can't be replaced. The military is very keen on the costs of your plan. My hope is you make your 20. I lurk @ e-r.org, and there are many military retirees there that can address you better there. You can also read the stories of people being pushed out a year or two early. Most of those makin their 20 were officers, usually nukes or some type of pilot.