Buying Back Active Duty Time as a Federal Employee: Lump Sum or Payroll Deduction?

Situation: I'm a 44 year old reservist with 20 years in, and I'll almost certainly be staying in the reserves for at least another 3 years. I'm also a federal employee with about 7 1/2 years in. I'm doing the buyback program for the 11 1/2 years I was on active duty, which will take me to 19 years in the Federal Employee Retirement System (FERS). For the curious, this does not affect my reserve retirement; I'll have both pensions eventually.

Other factors: I'm financially independent now with no debts.

Buying back my active duty time for the extra civilian pension will cost me about $20.7K

How should I pay that off? There are three options:

1. Pay it outright with a check. I have the funds available. On the upside, this would get me a 19% of my high three as a pension even if I got laid off the next day, and come next year, that would boost to 22% (the multiplier becomes 1.1% per year once you hit 20 years of federal service). On the downside, this costs me $20.7K now, which I would otherwise be investing for the next 15-25 years before I retire from working.

2. Pay it over time with a payroll deduction: I'll probably aim to pay it all off over about 10 years at $152/month or $2K per year if they don't charge interest on this. On the upside, this would keep my current $20.7K invested and growing. On the downside, I could be up a creek if my federal employment ends before then.

3. Make an initial payment now with a payroll deduction over time to finish it off. I don't see any unique ups or downs to this.

One big unknown about this is interest. The letter I got from DFAS doesn't say what, if any, interest gets assessed against the amount I owe. DFAS did include interest accrued to date, but it's not clear whether or not this interest will continue to be applied to what I owe as I'm paying it off. It doesn't specify what the interest rate was or is, either. I might try calling DFAS about this tomorrow.

My immediate inclination is to pay it all off now, not worry as much about getting downsized/laid off, and just have done with this, but I'm not sure how smart that is mathematically. I don't like living in a state of owing someone else if I have the means of paying off a debt, and this has led me to pay off even low-interest debts early, even though I knew that was mathematically suboptimal.

Happy to have your critiques, suggestions, comments, and opinions on what would be most optimal way to proceed.