Underpayment penalty

[progree]

11. Umm

I have taxable income in every quarter, but I send nothing in until December and then via withholding.

If it's wage or salary income, then taxes are withheld from your paycheck. So you've satisfied your tax obligation on a timely basis through withholding. I've read that withholding other kinds of income, like an IRA RMD that the IRS treats it as spread equally thoughout the year even if its made in December. (Not true for estimated tax payments, unfortunately).

I'm obligated to pay tax on that first quarter capital gain I had but I can meet that obligation in December and no 2210 is needed.

If you mean you had that capital gain in the first quarter of 2023, then I'm very surprised that you don't have to pay the tax on that until December 2023. I've never heard anything like that before. (Edit - unless you've already satisfied one of the safe harbor provisions, then no problem)

Another edit - and unless you can specify an amount of withholding in December on something like an IRA RMD that covers the remaining tax obligations for the year including that capital gains tax. I don't think either Vanguard or Fidelity lets me specify any percent or $amount of withholding, other than their default value, so I wouldn't be able to do that.