We're looking at more like $30k for two adults, no kids. I'll have to do math to figure how that works out.
Having no kids does make this plan harder in some respects. As a first cut, you'll get 20k between the standard deduction and personal exemptions for two people plus maybe another $3k for property taxes. That would leave 7k to be taxed at 10%, or $700 on $30,000 of income is an effective rate of 2.3%. Which isn't quite zero, but can certainly be improved by tax loss harvesting, a rental property, or a variety of other tricks.
So the plan would be first to save up five years of $30k spending in your taxable accounts*. The first year after you retire you either 1) roll over the $30k that's earmarked for five years down the road, and you pay the ~2% tax rate on it, or 2) roll over the $23k of tax-free 401k to Roth IRA conversion at 0% tax and plan to make up the other $7k with taxable accounts or old Roth IRA principal withdrawals (which are always tax and penalty free).
As to the $18k/year mortgage bill, consider the benefits of just paying it off. If you have enough money saved up to retire, you just need to decide if you lose more money by carrying the mortgage and paying (presumably low) taxes+interest on the payments or by withdrawing the money to clear the mortgage but then losing any future returns from leaving that sum invested in the markets.
*: by "first" I do not mean at the expense of maximizing your TSP contribution. Generally speaking you should be doing that first even if it means you can't save up 5 years of expenses in your taxable account, because the tax savings usually outweigh the 10% "penalty" for early withdrawal.