how to transfer low resolution photos ( optimised ) to the computer
If you deleted the photos from iCloud directly more than 30 days ago, and it appears you did if they're not in the Recently Deleted folder, then they're probably gone for good.

iCloud is a syncing solution and not a backup solution.  It doesn't store a copy of the pictures, in the way that using something like Dropbox does, it give you access to the pictures on all your devices.  At some point, you selected Optimize iPhone Storage in your iCloud settings.  Doing this only stores low-res versions of the photos on your device while the full, high-res photos are stored in iCloud.

If you saved any of those photos from iCloud directly to a computer, that would be your only remaining option to get the high-res versions.