A few Days ago, I was also searching for a solution to convert Outlook PST files to EML. I had tried the number of Manual Approaches but was facing issues while converting the attachments. Then one of my friends suggested me to use SysinfoTools PST to EML Converter Software (https://www.sysinfotools.com/recovery/pst-to-eml-converter.phpl). This utility helped me to export Outlook data files to EML with attachments without any hassle. The most amazing feature of the tool is that it is capable to migrate PST files to EML with attachments.
Here you can use a manual method or any third party solution to convert PST files to Office 365 mailbox without any hassle. There are various applications available in the online market, but it is a difficult task to test each tool. So I suggest you the best third-party tool such as Import PST to Office 365 Tool, which I used a few days ago, it provided me a fast and secure way to convert PST file to Office 365.