Admin approved file uploads
I believe what you wanted is similar to commerce file module where users can upload file and have configurable settings, where you can limit access duration, number of downloads etc. and about this one downloadable immediately, as soon as they uploaded via sites/default/files. Is this a big deal? yes it is big deal once you uploaded it in sites/default/files everyone can access it and download anytime without restriction you may violate some regulation. you should upload it outside Drupal where users can't access it via url. Where in commerce file already have this settings where you could place your file folder outside Drupal. I am not sure if this is already available in Drupal 8.