For what it's worth, 80% quality is already one of the default settings, "JPEG - Better Quality", and this may be what's chosen for most JPEGs by default; however, it would always be overriden by specific settings previously applied to a file, if any exist.
Also, the File > Save As… command would not necessarily be recommended for this as it has a known bug. It's ideal for saving copies of files in formats other than the file's current optimized output format. But if you start with a JPEG and then use Save As… to create another JPEG version, you're in for a surprise: Any settings you enter within the Save As options dialog will be ignored, in favor of the settings already entered in the file's Optimize/Image Preview dialog. So you'd be better off applying your "JPEG - 80%" setting within the Optimize panel and then choosing the File > Export command. (Or performing a similar operation within the File > Image Preview dialog.)
That said, I think you could possibly automate this process using File > Batch Process… Within its first dialog, choose "Include Current Open Files", and then click Next. In the next dialog, select and add the Export command and choose your desired setting ("JPEG - 80%") from the menu at bottom. Within the next dialog, click the "Save Script…" button and give your script a meaningful name, like "Export - JPEG 80%". Save the script within your application's Configuration > Commands folder.
You should then be able to run this command from within an open file (by selecting it from the Commands menu) or apply it as part of a batch process to a group of files.