@Chas R
You can't make contributions to your TSP account except via payroll deduction when you are employed by the government. Since you are not employed by them any more, you can't make new TSP contributions.
You are allowed to contribute to either a traditional (pre-tax) or Roth IRA even after you have declared retirement. You must have compensation from working, so even though the limit for IRA contributions over age 50 is $7000 per year, your maximum contribution would be $6000 if that's your income from working. TSP withdrawals are taxable income but they are not "compensation from working" and can't be contributed to any kind of IRA.
Turbotax does have a section where you can indicate that you made contributions to either a traditional or Roth IRA. You are correct that if you contribute to the Roth IRA, the wages will still be included in your taxable income for 2022, but you won't pay tax when you withdraw funds from the Roth IRA.
