Anyone got a best practice template for the order of a migration for a small business CRM and supporting applications?

The org I'm working with has a CRM (~4000 contacts), another external application that isn't linked other than through a link by email and then some social media accounts and a website.

Just looking for some pointers in terms of migration and how to phase each part. First should be the CRM data to the new CRM. Should the payments application be set up first and then connections to the payment portal on website be done first and go-live before cutting off the old payment portal?