I have experience with AWS and have already developed a WebTV. I can do a thumb from an uploaded video to the S3 easily in python or nodejs.
The stream would be developed using the S3 triggers for PUT or POST in a specific path (where the uploads would be made) would trigger a Lambda function that would generate the thumb and upload to the same path or other S3 path. Simple and functional. I have 3 projects running with similar structure.
Or if you do not upload directly to S3 and the file goes through your server the thumb can be done on your server and you would upload the video and thumb at the same time.