How are Seagate Firecuda 120 2TB and Barracuda 120 2TB for iMac?

I’m surprised you are hesitant on going with a PCIe/NVMe blade SSD. While some people have paid the price trying to force fit a M.2 SSD the OWC SSD is quite good. I’ve put in quite a few and have not had any issues with my customers. Often I get systems which have the M.2 SSD’s and find the issue is with them.

Availability via iFixit for the OWC Aura Pro X2 Blade SSD appears to be OK iMac Intel 27" (Late 2013-Mid 2015) SSD Upgrade Bundle (488GB or 1TB drive) Which is the size I mostly recommend for most folks. As you want it to be your boot drive and host your applications with about 1/3 of free space for virtual RAM, caching and application scratch space. iMac Intel 27インチ Retina 5K Display Blade SSDの交換

If you want a real Apple SSD here’s where I go BeetsTech for the PCIe 3.0 x4 interface PCIe/NVMe blade drives

• 512GB SSPOLARIS Custom Apple SSD
• 1 TB SSPOLARIS Custom Apple SSD

While the 2.5” SATA SSDs are also a good choice its I/O is limited! SATA III (6.0 Gb/s) is a lot slower than the PCIe/NVMe interface which gets you effectively 7.9 Gb/s with a lot less overhead than what SATA offers! Which is nothing to sneeze at! Most of todays drives are hitting the limits of the SATA I/O channel with its overhead you only get 4.8Gb/s.