Sadly your system does not offer the second drive connector of the blade SSD. For reference here’s what EveryMac states:
“*This model has a Serial ATA (6 Gb/s) connector for a 2.5" hard drive. If configured with a "Fusion" Drive or an SSD at the initial time of system purchase, it also has a PCIe connector (PCIe 2.0 x4 NVMexpress interface), but this connector is not present if the system only is configured with a hard drive at the time of purchase.”
So your only hope here is to upgrade the SATA HDD to a better drive iMac Intel 21.5" Retina 4K Display Hard Drive Replacement. You have two choices you can go with a 2.5” SSD or you can also go with a SSHD hybrid drive.
As you already have a 1 TB drive you’ll need to reduce how much you have if you have it already pretty full or think about going with a SSHD which will be cheaper with the larger storage Seagate Firecuda 2.5”. While not as fast as a SSD it makes up for it by being a faster disk drive 7200 RPM and the SSD cache will speed up the boot and running your apps.
As far as the SSD it too is a great way to get performance but you do need to be careful on how full you have it. You were thinking about a 500 GB drive for this drive to run well you’ll need to leave 1/4 of it free so the OS and your apps have free space to work with.
Your system does not offer serviceable RAM (soldered) so depending on what you bought your RAM maybe limited to 8 GB.