Desperately need help restoring fusion drive on a 2019 iMac.

Had a hard drive crash. Genius Bar indicated that they no longer repaired this model and recommended a hard drive. Replacement went well. But, initial install of iOS seem to be running only off of the hard drive. Nothing off the flash flash drive, which indicated on Disk Utility that the Apple SSD flash drive was "Uninitialized"

Some quick reading indicated that we needed to open a terminal prompt and enter a set of instructions that would tell the operating system to see the flash drive and the hard drive as one fusion drive. Enter the instructions exactly as indicated. It seemed to begin the process but then gets stuck. Please see the attached photos.

Our first indication that we had a problem was that when we went to install other than the original 2019 iOS, Catalina, it wouldn't allow us to select the single drive that appeared. Ultimately, with the help of Apple tech-support we were able to install a newly downloaded version of sequoia 15.3.2.

All files are backed up the Time Machine and this is a clean install.

How do we proceed from here? Do I try to run the terminal command again?

Any help would be much appreciated