You must make some kind of estimated payment. If you wait until April 2022 (the third option you mention), you will definitely owe underpayment penalties even if you pay in full at that time.
The only real question is, do you want to pay a lump sum in the quarter you make the conversion (payment due Sept 15, 2021 for income paid during June-August), or do you want to make equal payments (due May 17, June 15, Sept 15 and January 15, 2022). Since you already missed the first two estimated payment dates, you are probably best to pay it all by September 15 and use form 2210.
