I suggest that you also look at external storage solutions as a much simpler and cheaper alternative, with minimal performance sacrifices.
While my search is by no means exhaustive, I could not find instances of people installing a third-party PCIe SSD in the iMac 5K Late 2015. None of the websites Monomeeth suggested sells a PCIe SSD for the iMac 5K Late 2015, either. Unless compatibility is confirmed, I'd rather not spend money on a PCIe NVMe SSD (expensive), take my iMac apart (difficult and risky), only to find out that it won't work.
Even if there exists such products, they might be Apple proprietary and carry a hefty price premium. In contrast, USB 3.0 external enclosure and SATA SSD are both competitive markets. You have a broad range of products to choose from with wide availability.
If it narrows down to SATA SSDs, the performance difference between internal and external via USB 3.0 becomes very small. You are either limited by SATA 3.0 (6Gbps) or USB 3.0 (5Gbps). In return, you get a much easier upgrade. As you are probably aware of, the iMac 5K Late 2015 disassembly and reassembly is very difficult. I consider it a big plus not having to take an iMac 5K apart and put it back together, although you might enjoy the process and consider it its own reward.
If you have the budget and can find the right PCIe NVMe SSD, go for it. If not, a high quality USB 3.0 SATA enclosure and a good SATA SSD e.g. 850 EVO/PRO will make you just as happy while saving you a lot of time and trouble.