First of all can you clarify which application are you trying to host - Odoo or Magento?
If you are NOT sure of using Amazon EFS, then the other option is to use another Linux machine and this machine will actually act as your EFS. Hence, the data will now be available centrally and you can configure AWS application instances for auto-scaling. I hope you are getting what I am trying to say. The only drawback i see in this configuration is your new Linux image can be a single point of failure. On the contrary, you would get good performance as this Linux VM is in your VPC/local network.
Coming to the size of the instances - application & db, it really depends on the no. of requests that your application will deal with. I have done some level of research for Odoo but not about Magento, so I need you to answer my first question which I have posted above.
Regarding the Step 3 that you have pointed out in your post which talks about Amazon RDS, all that step is doing is cutting off the write capability to the database while you are performing the migration. So if you have an application hosted on the Amazon Linux VM, you can just shut the application service down so that it doesn't connect to the db at all. And side by side, you can initiate your db migration by connecting to the RDS instance. Of course, you will have to plan for maintenance period.