Yes, that would work.
If you have a lump sum income (conversion) in December, the IRS is going to want to see quarterly estimated tax payments that were due April 15, June 15, Sept 15, and Jan 16, 2026. You already missed the first two payments. You can make estimated payments in Sept and January, and then include the penalty form 2210 with schedule AI with your tax return to show the IRS that even though you did not make payments over the whole year, your payments were appropriate for your income in each quarter.
Because withholding is assumed to be evenly spread out over the whole year, even if it is not, you could increase your W-2 withholding to cover the tax, by having enough extra taken out over the last 4 months to cover the conversion taxes.
A third option is to have taxes withheld from the conversion. Suppose you convert $20,000 and have $5000 withheld. That will satisfy the IRS because both income and withholding are assumed to be spread out over the whole year. Then, you can send the Roth IRA a check for the $5000, and tell them it is also a conversion/rollover. They don't need to know or care that this is part of the same conversion. You must send the make-up check within 60 days, and you can only use this method once per year. But if you have the cash available to make estimated payments, this is an alternative way of getting your taxes received by the IRS in a way that avoids the penalty calculation.
Finally, if you converted in the beginning of the year (Jan-March), the IRS will want to see 4 equal payments of 1/4 each on those same dates, April, June, Sept and Jan. That would allow you to invest the tax money in the mean time.
