I suggest MailsDaddy PST to MBOX converter, which is the best GUI based application. the tool has in-bulit lot of features and its interface is user-friendly so there is no technical knowledge required to use it. As well as you can use bulk export option and migrate all data at once. its Demo version is available on official website.
Refer: https://www.mailsdaddy.com/outlook-pst-to-mbox-converter/