Hi,
Regarding SSD NVMe upgrade options for intel iMac:
• If your iMac is 27” Late-2013 or newer, it has PCIe slot (both “FusionDrive” and “HDD” models).
• If your iMac is 21.5” with FusionDrive from Apple, it has PCIe slot with one small Apple SSD.
• Apple PCIe slot is compatible with Samsung 970 Evo Plus M.2 NVMe SSD (firmware updated) plus Sintech adapter. Adata Pro NVMe SSD is cheaper and good too. Both are twice as fast compared to the original Apple SSDs.
• macOS High Sierra required.
(Upgrade Macs is my favorite work at www.globalmac.cl)
Good luck!