Is there a simpler method than annualizing? If you do not meet any of the safe-harbors, no. The US tax system is generally pay as you go, but to simplify things for the majority of individuals the IRS provides a default method that treats your income, deductions, credits and tax withholding as received uniformly throughout the year. Note, however, that estimated tax payments are never treated as received uniformly throughout the year. They are treated as paid when actually paid. Of course for those who receive a large amount of taxable income late in the year the default method doesn't work and it must be abandoned in favor of annualizing income.
What you could have done to avoid needing to annualize income would have been to have taxes withheld from the Roth conversion equal to the estimated tax payment that you would otherwise have to send, then complete the Roth conversion of the withheld amount indirectly by substituting other funds. This avoids the need to annualize because tax withholding is treated by default as having been withheld uniformly throughout the year. Such a method can also be used to manufacture tax withholding if you would have underpayment for earlier quarters even without the Roth conversion. The alternative is to anticipate your Roth conversion and increase your tax withholding from other sources or to pay your estimated taxes evenly over the four tax quarters.
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