I think you have the wrong idea on what a Fusion Drive is. Your system has only two drive ports a SATA port and a PCIe blade connector. Both ports are used to create a Fusion Drive set. If you take out your HDD to replace it with a SSD drive you’re not creating a dual drive config which is needed.
To create a Fusion Drive If your system has a HDD drive then your only choice is to use an Apple Custom PCIe/NVMe drive for the PCIe connector on the backside of the logic board.
As for a Fusion Drive using such a large SSD This would be a waste! You would want something around 64/128 GB in size. Or, leave the SSD in a dual drive config making the SSD drive your boot drive and then make your HDD as your data drive.
Keep in mind if you are planing on swapping out the SATA HDD for a SSD you are limited to the SATA III (6.0 Gb/s) throughput of the interface whereas the PCIe interface is almost twice the throughput which the Apple SSD drive can leverage.
Given how much harder it it to install I would strongly recommend you just get an external Thunderbolt drive at that would gain more performance than what your SATA based SSD would have and besides you wouldn’t loose the storage space of your current HDD (or need to open the system). Here’s one that fits the bill: OWC Mercury Elite Pro Dual