Let’s review what this system offers for a Blade SSD interface referenced here [https://beetstech.com/blog/apple-proprietary-ssd-ultimate-guide-to-specs-and-upgrades#hdr-16]
[quote|format=featured]The 21.5″ iMacs and the 27″ from 2013/2014 only support PCIe 2.0 x2 connections. All iMacs of this era support NVMe, but the speed increase is more limited in the 21.5″ iMacs and the 2013/2014 27″ iMacs, where the two channel interface acts as a bottleneck.[/quote]
So you will need a two lane PCIe/NVMe SSD and all of Apple Blade SSD are custom you either need a real Apple SSD or one of the pin compatible SSD’s like from OWC. I don’t recommend using a M.2 adapter as they have been proven not to be that reliable, to add to that the depth you need to go to even get to it makes having to go back in not worth the trouble.