Use the following at your own risk. I write it from my best recollection but guarantee nothing.
Kies is a turd, so I'm not surprised it gives you bad information and fails to help.
I think you misunderstand what a factory reset does and does not do. Doing a factory reset does not restore any bloatware or any other app you may have removed. Factory reset wipes your data partition, so all of your settings are gone. As a result, the next time you boot the phone, it has no settings and consequently does a new, out-of-the-box startup sequence. Factory reset does not touch the system partition, which is where the bloatware resides.
Getting full stock 4.3 (JellyBean) back on your device would require the same basic steps as putting 4.4.2 (KitKat) on it directly, so you might as well skip the 4.3 step and instead just do 4.4.2. Note that once you go to the AT&T/Samsung-approved 4.4.2, you cannot go back to the AT&T/Samsung-approved 4.3, because AT&T/Samsung locked the bootloader such that it does not allow this downgrade.
To put either version (4.3 or 4.4.2) of the approved stock ROM on your phone, you will need to download Odin and use it to put a customer recovery on your phone. You can find many posts telling you how to do that. The quirky part is that, after Odin pushes the custom recovery onto your phone, you have to pull out the battery, instead of letting Odin tell the phone to reboot. If Odin tells the phone to reboot, the phone will put its stock recovery back in place just before the reboot, and you will be back at square one. I am most familiar with TWRP custom recovery and have used it to install multiple ROMS, including custom ROMs and the stock approved ROM, on my i747 (AT&T S3).
Once you have installed a custom recovery, you will need to download the full stock AT&T/Samsung-approved ROM installation zip from the web. I cannot post a direct link to such a download, because I do not have enough posts on the forum yet, but it should not be too hard to find. You then can use TWRP recovery to 1) first make a backup of what you have now, just in case the new ROM does not install, 2) wipe the system, data, and cache partitions, and 3) install the stock ROM. Copy the stock ROM zip installation file to your phone before starting the three steps listed above. Note that your apps, settings, logs, text messages, personal information, ... everything will be lost when you do the upgrade in this way. If you connect to the same Google account after the upgrade as you used before, it should put the apps and some settings back. Call logs, SMS, and some other stuff will not be restored, however.
Good luck.