Hi,
Even if you are using the schedule action you need to write a python code to achieve this need. Below is a simple app for you. We can upload documents to the employee in the employee page and can set the expiry date and also email notifications settings. You can upload the visa document as a document of the employee and set the expiry date and also put the email notification type and mention the before number of days(30) that you need the notification.
https://apps.odoo.com/apps/modules/14.0/oh_employee_documents_expiry/
If you need to write the scheduled action then check the following in the above app
1.record with id employee_data_reminder which is the scheduled action record
2.function with name mail_reminder which is the python function used in scheduled action
Regards