SAP Marketing Cloud(SMC) to Marketo : Migration Approach

Hi All,

We have requirement where we need to integrate SAP Marketing cloud(SMC) system with Marketo , could you share some best practices/reference doc.
Here is what I think of , but please let me know what is the approach/sequence of best practices that we need to follow ?

1) We have 15million contacts in SMC that we would like to migrate it to Marketo
2) Before starting of actual migration what are the pre-requisties that we need to consider?
3) What is the sequence of data migration that we need to follow from SMC to Marketo?
4) we want to migrate : Contacts , Interactions(Activities) and Events related data from SMC to Marketo
5) In SMC we have structured data for contact , interaction & events. where as in marketo it is flat structure. How to handle this?
6) Considering the volume of records for Initial data migration from SMC to Marketo : what API to be used either REST or BULK API or Manaul File Upload?

Please share the your lessons learnt during the migration to marketo

Thanks

Sai