How to generate different image styles at upload?
Ah yes, you are right.


  Image derivatives will be automatically generated once requested and not-yet existing.


That's a quote from my own answer to a similar question. I guess you could try to hook into the upload somehow like Image Style Warmer does it, or simply try that module out yourself.


  Synopsis
  
  The Image Style Warmer module provides options to create image styles
  during upload or via queue worker. So configured image derivates
  already exists when they are requested.