You’ve got an interesting problem. For your laptop, there are conflicting specifications, as if there was more than one model released under the same model number with different specifications. So I don’t know what to recommend for an SSD. A direct replacement would be m.2 SATA format, which would make zero sense…
But first things first. Why would you want to replace your SSD? 128GB should be more than enough to store Windows and maybe one or two games on it. There really shouldn’t be anything else on your boot drive. If you are storing everything on drive C, then you don’t need to buy a new SSD…you simply need to do some file maintenance. Start storing most of your stuff on the other drive…the 2.5″ drive.
Most of the major brands (the better brands) of SSDs come with cloning software (or a software key) so you can download cloning software and clone your drive to a new one. HOWEVER, you DO NOT WANT TO DO THAT.
Your SSD that you are talking about, 128GB Sandisk Z400S, should contain just Windows and not much else. If you “clone” that, then you are taking all the junk files and junk registry settings to the new SSD, which will greatly decrease the performance of your computer. Your Windows 10 COA should be stored in the BIOS for that computer. So a better plan is to make a bootable USB thumb drive and boot that to install Windows 10 on the new SSD.
But I don’t understand why you would want to replace the original SSD. That should be perfect for your computer, for as long as you own the computer.