Over half my income gets paid out in a variable annual bonus, I literally won’t know what it will be until almost a quarter of the way into a year, and the flat 22% withholding is indeed usually too low and would leave me subject to under withholding penalties many years. As far as I’ve read, anyone can make estimated quarterly payments. There’s definitely requirements about when you make the payment (eg needs to fall within some period of the quarter you receive the income) but I make sure to send the payment around when I receive the bonus. My withholding is sufficient otherwise to cover my non-bonus pay.