Upgrading an 2017 iMac 27" with an off the shelf NMVe m.2 SSD + M.2 Apple PCIe adapter is definitely possible.
I am amazed how much confusion information is on the internet about this topic. Even from OWC, which only wants to sell a SATA upgrade kit for the 2017 iMac.
So I thought, try and find it out.
I did my 2017 iMac with a EVO 970 NVMe SSD + Sintech M.2 NVMe to Apple PCEe adapter.
I only recommend the Sintech adapters, others are known to have issues.
I have some images on instagram where I did my upgrade.:
I am happy I did it and got rid of the FusionDrive setup.
As a note, if you plan to replace the SATA drive with an SSD, which I also did then "No" you do not need the overpriced and useless thermal Sensor from OWC either.
I recommend the regular EVO not the EVO Plus or EVO Pro.
If you still insist on using the non standard EVO then Samsung released an firmware update for the EVO Plus that fixes issues on Mac, you need to upgrade it first using a PC before putting the drive in your Mac.