Migrating a 2015 iMac to external SSD for the OS

I'm finding my iMac slow and wanted to use an external SSD drive instead of the Fusion drive. I've watched numerous videos and done this on a 2011 iMac and 2013 mac mini, so I did think it wouldn't be that hard. The 2015 iMac is USB3, but with no Thunderbolt or Firewire options for disks. I got a WD Blue 1TB USB3 and a quality enclosure to host the drive and a decent cable.

I've tried formatting the disk with Mac Os Extended (journaled) and also APFS and used Carbon Copy Cloner to copy the data over. It gripes about OneDrive having "open" files, but completes and the drive is viewable from the OS (Big Sur).

However when booting with "option" I can only see the original internal drive and time machine drive, and System Preferences > Startup disk, there is only 1 volume (the original hard disk). I have tried 2 different cables between the Mac and the disk as well as swapping between different formatting options. All with the same result.

I did try Recovery mode and tried to back up the OS to the SSD, but that failed as well.

Any help gladly accepted