I'm not sure if you can implement the spatie/image inside the store methods. But you could try the code I posted above. Explanation of what it is doing: Saving your image -> Get the image -> Resize it -> Override the original image with the resized one image. Probably there is way more properly way of doing it but I can't help you with more than this.
@Laralex don't get in the habit of just using just a timestamp for the filename. 1 second is a long time and it's quite easy to get separate users uploading at the same time. Also, it definitely does not work if you upload multiple files.