The current penalty is 7% interest on unpaid amounts. Back when interest rates were low, it was 2-3% which you could easily beat, but 7% is tougher.

You are mistakenly conflating interest (which can go up or down every quarter and is charged daily, like an unpaid credit card balance) and penalties (which are a fixed amount per month set by law up to a maximum number of months). Most importantly, both of them apply if there is a failure to pay (FTP) by the original due date.

I am aware there is a difference.

Most people would fall under the "underpayment" scenario since taxes are deducted on their W2, which is just interest on the underpayment.