Why do you want to import 13K of users in your app in the first place?
“ you want to have the data of the users in your app”
My guestimation is that you want to have the data of the Employees in the app, and that these possibly need to login.
If above is correct, then split the employee details from the account/user details. Then you can import the AD users as employees, but they will not have an account. And thus your license is not a limitation for you.
Also in that way you split the login function from the process function.
Matching of the account & employee can be done on the first login of the user. Then the account is create and can be matched using the email address for example.