The numbers that K M W has provided to you show that it will be impossible to convert the entire $2M to Roth by age 73 without some ate 32%. Staying out of the 32% tax bracket would require converting no more than around $150k per year. Pushing AGI beyond $200k will also trigger Net Investment tax.
Nothing prevents you from converting similar amounts in and after the year that you reach age 73 as long as you complete your RMD first.
Don't forget to consider state tax rates as well and as well as any effect that increases in AGI have on the taxability of Social Security benefits and, if you have Medicare Part B or D, Medicare IRMAA.
