Photoshop’s “save for web” function (as far as I know) does not resize an image, it compresses it. And it’s very unlikely to get a 300kb image out of a 6MB image, but yes, I get what you mean. What you’re looking for is an image optimization plugin like EWWW Image Optimizer or Imagify.
I also develop EWWW, and I can tell you right away the only way you are going to see dramatic savings like that is by paying for the lossy compression (with either plugin, since Imagify has a 2MB file size limit on the free plans). The average lossless compression with EWWW is around 9%. In your case, it sounds like you have someone uploading stuff straight from a camera, so you might get a little more shaved off by ditching the metadata with plain lossless, but it still probably won’t be more than 15%, whereas lossy can get you 50% average savings.
And of course EWWW will also do all the same things Imsanity does for resizing, so if you resize AND compress, you could very well get down to 200-300kb on that 6MB image ??
