There are several adaptors at Ebay or Amazon for around 10-20 Euros.
I'm using a sintech adaptor to use a normal M.2-NVMe SSD (Samsung Evo 970 Plus) in my 2019 iMac. No problems at all.
For Samsung SSDs, be sure, you have the very latest firmware on your SSD installed (i'd used a Windows laptop and an external enclosure for updating the firmware) and you will need at least macOS High Sierra to support NVMe-SSDs.
For frequent firmware updates, i have a bootstick with Bootcamp/Windows, because absolutely no manufacturer supports firmware upgrades via macOS and only Crucial(Micron) is providing a Linux based upgrade volume.
Sadly Apple is the only source for the PCIe/NVMe x4 blade SSD's. While you could put in a PCIe/NVMe x2 blade SSD you won't get the same throughput.
it seems that patience is a virtue!
I changed the disk 2 TB for a 2.5" SSD and I boot on the 128 g ...
there will be a builder to make us that !!!
like 512 or 256
Thank you for your answers
I have the same iMac as you just i5. OWC have what you want. I’m buying their OWC Aura Pro X2 2TB and OWC Mercury Electra 6G 2TB SSD. The 2TB SSD as my OS X (Mojave) drive and the 2TB PCIe Aura as my audio recording drive.
door Adrian Swift