Replacing Fusion Drive in 2017 and 2019 iMacs (Leaving New NVME Only)

I don't have a problem with the physical replacement which I know is involved, but I wanted to make sure I understand the technology of the Fusion Drive.

1. I am under the impression that it is okay to take out both drives, and just replace the NVME with a 1TB (blade) NVME drive, leaving the 2.5" drive out of the equation completely. Is this correct?

2. Is there any reason to go through the process of splitting the drive in the software if I will not be using either of the two original drives again, or will that have ramifications within the system after I install the new drive? I guess I'm asking if it's OS level changes or system changes. I've not looked too far into that aspect.

Thanks!