With a large Roth conversion, I assume that, if nothing is withheld for taxes, the total underpayment for the year will be more than $1,000 and more than 10% of the tax owed. If that's the case, the penalty will be based on the quarterly underpayment of the amount needed to meet the safe harbor based on your 2023 tax liability.
Opus 17's suggestion #2 is probably the easiest way to avoid any underpayment penalty. Just be aware that the amount withheld for taxes will be uninvested until the check to complete the entire conversion is deposited into the Roth IRA.
