That's how aliases work. Pretty much all of the services listed here - https://purchasewithpurpose.io/category/email/ - have them. You can have multiple email addresses, but yet they all go to the same inbox.
As a strategy to minimize your effort in the migration, start by forwarding all your Gmail accounts to the corresponding inbox and/or alias. You can then start using the new service right away and take as long as you need to migrate your accounts across.