Exporting images in full resolution from photos APP...

I need to click on each image and then 'edit' for the high res to preview to be displayed instead of a pixelated preview.

Have you tried to rotate the photo back and forth instead of going into the edit mode? Rotating can be done while viewing the thumbnail. Just select a few thumbnails at once, then use the command ⌘R to rotate in one direction, wait for the thumbnails to be recreated, then rotate back with ⌥⌘R. If it works, it will be quicker than going into Edit mode for every photo.

Some cleaning applications are simply not safe to use. Apple is warning again programs that modify the Photos Library directly, see: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

You are saying exported at "Maximum JPEG Resolution". But what are the settings for the size? Is it set to the full size? The default is "Medium", and that is only suitable for stamp collectors.

Did you use an external editor to edit the photos or edit them on your iPhone? An external editor may have changed the pixel size of the edited version, but Photos will still report the original size.

Another possible reason for image files, that are exported at a smaller size is running cleaning applications like "Clean My Mac". When I tested the trial of "Clean My Mac 3" on a small test library, it had removed the high resolution original image files and replaced them by the smaller, edited versions, and there has been no way to recover the high quality originals from that library.

I'd be interested to know whether the result is different if you drag the images from the Photos app into a folder, e.g. on Desktop, rather than using File > Export.