I think Shopify postponing it is a good idea, but as you have no guarantee they will do that, it would be better to migrate sooner rather than later. There have been tons of people who have migrated without any issues (myself included), and waiting is only going to make issues for yourself and your merchants. If you do it correctly there will be no issues, but waiting means you have less time to implement and validate, which could lead to mistakes.
I would do the migration slowly over time, starting with new merchants and migrating a few each day.