You are age 45 to 55 and served for 20 to 30 years. Depending on your future income needs, your DFRDB benefit may already be sufficient to support your full retirement.
Please also use i-Estimator to project your potential DFRDB benefit.
Higher levels of tax must be paid on commutations (lump sum withdrawals) received before age 55 than taken after age 55. Such payments can also be rolled over to defer a tax liability. You should, however, seek professional financial advice.