You are correct. No underpayment penalty will trigger for you.
It is not necessary to send any estimated tax if your withholding from your employer, in 2021, covers the necessary amount of tax liability on your 2020 tax return. Withholding is considered paid evenly throughout the year regardless of when it was withheld. In your case, this is irrelevant since it was paid in the first quarter anyway.
A penalty will not be assessed by the IRS based on your current situation. As the year moves forward, you are all set and do not have to pay any estimated tax regardless of any change in income for 2021.
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