When the IRS files an SFR, they use your information returns to build a return with the worst possible assumptions. Single filing status, standard deduction only, no credits. The assessment is almost always dramatically higher than reality.
File the actual return. Even years later, you can file and request audit reconsideration. The IRS will adjust the assessment downward if your return shows a lower liability. There is no time limit on reducing an SFR assessment, but there is a three-year limit on claiming refunds. We handle SFR cases regularly.