What choice about SSD I can replace?

Dear iFixit

My name is Ethan, here's a question I'm confuse few days

I own a newest version of iMac 27" (2015 late) 5K display, i5 1TB Fusion Drive.

could I replace the Fusion Drive( a PCIE SSD with a 7200RPM HDD 1TB)

to one single PCIE NVMe Gen 2 or Gen 3 x4 256 or 512GB?

if I could and follow how with iFixit tips, what choice of SSD I have?

(I mean it can really working on the New version of iMac in 2015 late)

by PCIE NVMe gen 2 (ro3? )x4 SSD (seems the performance should be Write 1500mb/S

and Reading 1800mb/s around.)

Please kindly reply, Thanks a lot!

Ethan Chang