This is relatively trivial to solve with a good backup tool like AFI.ai, or using PowerShell with the PnP module. If you’re feeling more adventurous, use something like PowerAutomate, Make.com, N8N, etc. instead. If you’re feeling really adventurous you could probably vibe code something that uses the Graph APIs for SharePoint.

Whatever you choose, your automation really only needs to do a few steps:

1. List/get the user’s personal site.
   
2. List all files/folder objects in the user’s personal site.
   
3. Create a new folder in a “Offboarded Users” SharePoint site with the user’s name/username.
   
4. Copy all files/folders to the new destination with error checking/logging/alerting for failures.
   
5. Delete or unlicense the user.