@user17749670393 right - IRS will assume your income (and any withholding) is spread out evenly over the year unless you file the Annualized Income method (more useful for Roth conversions later in the year, not your situation). Likewise any estimated tax (ES) payments to meet safe harbor need to be spread out over the year by quarter, you can't just pay it all at the end even if you meet the total it has be timely. One of the few things that has specific dates in your tax return are the dates of ES payments.
So yes you can avoid penalty by paying 100% of last year's tax (110% if AGI > 150k or 75K if filing MFS) either thru withholding or "timely" (quarterly) ES payments. Since Q1 ES is due in April when you file your 2025 return, these ES payments should be fixed and known for that deadline (depending anything you have with withholding). Then the timing and amount of any income through the year will not matter, but remember to set aside the remaining tax due in April 2027.
When you prepare your taxes for 2025, Turbotax should produce ES vouchers for 2026 by default based on last year's tax and assuming any withholding in 2026 is the same as 2025. In Other Tax Situations / Form W4 and Estimated Taxes you can review other options and provide estimates for 2026 but if you are certain that the Roth conversion will increase your income such that 100% of 2025 tax is lower than 90% of 2026 tax then you should be all set using prior year tax.
Finally try to avoid sending checks and ES vouchers, if able you can pay directly at irs.gov.
Thinking ahead to 2027 tax year, if you had an unusual income increase in 2026, then for your 2027 safe harbor the 100/110% of 2026 tax may not be optimal for you and you may need to track against paying 90% of 2027 tax.
