128 SSD replace - mSata/M.2/Proprietary apple port?

The Apple blade SSD drive is a custom part. You'll need to replace it with either the Apple OEM unit or a 3rd party unit thats designed for your system.

You'll need to take the system completely apart to gain access to the SSD drive. Review this IFIXIT guide: iMac Intel 21.5" EMC 2544 SSD Replacement.

Opening up the newer 'Thin Series' iMac's are a bear to open! And you risk damaging the display assembly if you are not careful. Make sure you follow the IFIXIT guide to the letter as it doesn't take much to damage the display. Don't forget the additional amount of work it will take to even get to the SSD. You may want to consider just replacing your current HD with a SSD and leave the two SSD drives independent.

The adapter you've pointed to in the link allows one to reuse the Apple Blade SSD as a SATA 2.5" drive. Some people use this adapter in an older Mac Pro or Mackintosh system so they can recover customers data from a dead system.

Moving on here ...

Sadly Apple never offered a 1TB blade SSD drive for this system. But you're in luck here! OWC does offer a 1TB SSD unit that will work in your system. Here is the part: Aura 6G Solid State Drive. So you're half way there. Do take note the SSD is not compatible in slower i5 system.

As to your current HD & SSD setup you'll first need to break the Fusion drive set before you pull things apart. Follow this: How to split up a Fusion Drive.

Frankly, I think you'll find getting an external Thunderbolt RAID'ed drive a better solution than using the internal HD as your backup.