Frankly, I would just get the blade SSD only config (as large as you can afford) and buy an external RAID SSD drive as it will be massively faster than the SATA SSD. Just look at the performance differences and think about the cost and risks of opening your system.
If you do go with a SATA SSD you would need to add the SATA/Power cable (which is not available yet) and will need replacement adhesive tapes [gekoppeld product ontbreekt of is uitgeschakeld: IF174-005-1] following this guide iMac Intel 27" Retina 5K Display Display Replacement. You will need to pull the logic board forward (not fully removing it) to gain access to the header which is on the backside to plug in the SATA cable.
I don’t recommend using M.2 drives with the adapter in iMacs given the amount of work just getting to them. At least in a MacBook Pro its a lot easier to pull it when it fails (which it will) Stick with the original Apple blade SSD, yes its more expensive but you just bought an expensive system!
Think of it this way… You just bought a beautiful Bugatti would you take it to the local Jiffy Lube for an oil & filter change? I think not! They wouldn’t have the racing oil or the correct oil filter your car requires.
Please don’t mess up your system thinking you can do better. Get the best Blade SSD system you can get and then go with the external Thunderbolt3 RAID drives for the speed and the deeper storage. I’ve helped setup both audio engineers and video editors with Mac’s. This is what they go with and often times max out the RAM in your case your system can go upto 128 GB.
The Thunderbolt3 ports are very fast! Each port is about 3 times faster than the internal SATA port. so that's not the issue here. Are you thinking USB connections then that would be slow!
Fusion Drives configs are slower! And you gain nothing when setting up a Fusion Drive with two SSD's! In fact you slow your system down Vs leaving the drives independent.
Stick with the largest blade SSD you can go with and then if you really want deep data storage internally put in a large 3.5" SSHD drive as the cheapest solution. Then jump to a large SSD (in both we are using them independently not doing any fusion setups).
Lets dig a bit deeper here on external drive configs within a multi-drive Thunderbolt3 case (RAID)
The setup of the drives them selves then becomes the bottleneck. So if you plugged in a single SATA HDD or SSD into the case then its no slower than what your internal SATA drive would be.
Now what if you use a faster case interface in the case, lets say a PCIe/NVMe with M.2 SSD's now just a single M.2 drive could be as fast as your internal Apple Blade SSD! But, now let’s add three more!! Now we are three to four times faster than what your internal blade SSD!!
Still not fast enough or deep enough storage!
OK, So now add in a second Thunderbolt3 RAID box and then stripe the data across both boxes you'll get close to eight times faster than your built in blade SSD!
As you can see a good RAID box even with just one drive is still better than using the internal SSD.
If money is tight get a single USB-C SSD drive. Get either one of these great drives!
I have four T5’s which are just great! The new X5 is still faster! As the flash chips are internally connected via PCIe/NVMe, access of the older T5 is with the slower PCIe/AHCI connection which is still faster than SATA!! Both are still faster than the best internal SATA connected drive you could get.