iMac 2017 5K SSD Upgrade

Apple was not doing PCIe 4.0 in 2017 and the SSD connector is buried beneath the motherboard, so you would need to entirely disassemble the iMac to access it. Probaby the best you could hope for in terms of eliminating damage risk, your time, and compatibility issues is to go for an external SSD drive and enclosure connected to your Thunderbolt port — either as a Thunderbolt drive, or a USB-C 3.2 Gen 2 drive and cable.

OWC has solutions for this. Yes, there are NVME PCIe 4.0 external drive enclosures that would accommodate PCIe 4.0 2080 cards, but they may not be compatible with the Mac or the operating system.

Although I could, I am not using my external USB-C 3.1 Gen 2 Crucial X8 drives as bootable operating system drives, but rather as Time Machine drives. These drives are rated at 1050 MB/sec. I am using one as a container for my several Parallels Desktop 18 guests connected to the TB3 port on my 2020 Core i7 iMac. That iMac shipped with an internal SSD, so no need for a replacement. Crucial has these X8 drives in 1, 2, and now 4 TB capacities.

Ajinkya_Pandit wrote:

So now, while installing the SSD, Can I install NVME PCIe 4.0 SSD like Samsung 980 Pro in place of that Blade SSD?

yes

Also, is it possible to install the NVME PCIe 4.0 in place of 1TB HDD keeping the 27GB SSD as it is?

since you want maximum speed, I would personally replace the "blade" with the highest capacity PCIe SSD as I could afford and run macOS from that. and then you can use the HDD portion as supplemental storage.

but for an internal SSD I would recommend buying it from OWC. Macs can be fussy about which drives will work in them.

and I neglected to inform you that the HDD portion is SATA, not PCIe. adapters can be had for the Mac Pro, but I do not think there is one that would work in an iMac.