If you pick through this thread you will find the answer to how to prevent the wipe after update. The bottom line is watch your phone closely as it updates. Eventually you will see a line to the effect of "Script succeeded: result was [/system]". As soon as you see that, press and hold power, home, vol+up, vol+down until the phone reboots. I've done this a few times and it works. When the phone boots for the first time, it will say Nougat updating and will be sluggish for a sec but it will settle down. This was discovered and is described in this post:
https://forum.xda-developers.com/showpost.php?p=71758322&postcount=255
Also, samsung's cloud backup does a pretty good job.
To find the files, I brute force search for them based on the naming conventions that user khaliullov spelled out on a russian website. Basically, the filenames contain the android version, the bootloader version, the year and the month and the version of the update. I cycle through these by hand once a month looking for downloads with any size. ATT has been updating monthly pretty well. Occasionally, like for the December update, there will be numerous versions of a given month's update at which time one needs to be careful to get the non-beta and non-alpha versions as these may not have an upgrade path.
Cheers