This is a unique use case. Creating thumbnails when an asset is uploaded is a typical way people create thumbnails for assets.
Read this community article - it talks about a DAM media handler that shows you how you can dynamically create thumbnails.
https://helpx.adobe.com/experience-manager/using/damhandler.html
Maybe you can look at extending the video component. Take a look at this example:
https://helpx.adobe.com/experience-manager/using/creating-interactive-video.html
This shows how to display information at specific time intervals in the video. Maybe instead of showing information (as shown in this example), the component can invoke a back-end service that creates thumbnails at specific time intervals.