Data loss prevention

The most important factor is to get the data off those local machines. The best way to do this would be to promote SharePoint Online, you'd be a perfect use case for SharePoint Syntex as you can pull the data from those documents automatically and make users lives easier in the process: https://adminseanmc.com/2020/10/05/sharepoint-syntex-unlocking-the-power-of-your-data-with-a-form-processing-model/

I'd recommend looking at protecting the documents with sensitivity labels, you can event automate this based on content if you go for information protection plan 2: https://docs.microsoft.com/en-us/microsoft-365/compliance/apply-sensitivity-label-automatically?view=o365-worldwide#:~:text=When%20you%20create%20a%20sensitivity,use%20each%20of%20your%20classifications.

If you can't get the data into SharePoint then you have the option of deploying the unified labelling scanner to the machines to scan and protect data on them based on content: https://docs.microsoft.com/en-us/azure/information-protection/deploy-aip-scanner

To automate upload to SharePoint you could use a simple Flow to extract attachments from email and post to a SharePoint document library where you could have Syntex parse the relevant info from the documents automatically.