Is one big Roth conversion or several smaller annual ones smarter?

Usually several smaller conversions win. Spreading conversions across multiple years lets you fill up the lower tax brackets each year without spiking into a higher one, keeping your average tax rate on the converted money down. A single large conversion can push a big slice of the money into higher brackets, trigger Medicare IRMAA surcharges, cost you ACA subsidies, and raise the taxable portion of your Social Security. The exception is a rare deep-income year, say early retirement before Social Security and RMDs, where a larger conversion at a low rate makes sense. Plan the sequence with an eye on bracket thresholds. Model different conversion sizes at wealthserene.com/tools/roth-conversion.