seems generally you are following the right process, and using form 2210 to communicate the back-loaded tax liability and corresponding estimated tax payments. I am going thru same process for a Roth conversion and managed to minimize the penalty but not eliminate it because of underpayment in Q2 which can't be fixed in Q4. Without seeing all the details it's probably hard for anyone to explain why you are still getting a penalty for your situation. In the TT forms there should be a page "2210 Penalty" which shows how the estimated tax payments are being allocated against the required installments to calculate the penalty, I found this helpful to explain the residual penalty.
