Software update only indicates updates for the OS you are running at the moment, as there are no more updates for 10.8.5
then that is why you see nothing. Going from one OS to another OS is called an upgrade, and cannot be done through the
Updates section of the App Store, or Apple Support Downloads site.
The updates that you downloaded previously like 10.9.5 are only updates and you must have an earlier version of Mavericks installed for those updates to take, as you are on Mountain Lion 10.8.5 that could never work. Mavericks full install version is no longer available for download.
A MacBook Pro mid 2012 can install the latest version of the macOS which is Mojave 10.14.2, and you can only upgrade via the App Store, How to upgrade to macOS Mojave - Apple Support
Going from Mountain Lion 10.8.5 to Mojave 10.14.2 is a huge leap, so be prepared for some major changes.
Firstly check that any third party apps you use have updates available so they will continue to function in Mojave, you need to check with the developers of those apps.
Also check that any printers/ scanners etc have updated drivers and software available so these will continue to function, again check with the manufacturers of any hardware products you use with your Mac.
There is no need for you to step by step upgrades. Mavericks and Yosemite are no longer available for download.
El Capitan is still available, but as your Mac can install newer OS's you will be refused it.
Sierra and High Sierra are still available for download if you wanted to try those before upgrading to Mojave.
How to upgrade to macOS Sierra - Apple Support
How to upgrade to macOS High Sierra - Apple Support
