REVert from APFS to macos Extended journaled

Repartitioning the drive from APFS to Mac OS Extended will erase all data on the drive and it cannot be done to the drive the Mac is booted from. You must reboot from another drive or boot in Recovery mode.

Why do you want to change the format?

Every macOS version since Catalina requires the internal drive to be formatted APFS.

Make sure you have created a bootable USB installer before you go any further, and confirm that USB installer does in fact boot the Mac properly. Also make sure you have backed up your data because it will be wiped when reformat your internal startup drive.

Create a bootable installer for macOS - Apple Support

Back up your Mac with Time Machine - Apple Support

Erase and reformat a storage device in Disk Utility on Mac - Apple Support

FWIW, I have the identical iMac, albeit with a 1TB mechanical drive rather than a 3TB unit, also set up as Fusion Drive, and it works perfectly formatted as APFS. So the problem with your system was not due solely because your Fusion Drive was formatted as APFS. But if Mac OS Extended works better for you, then that certainly should be just fine.

Note that when Apple says that Mac OS Extended is recommended for versions of macOS from 10.12 or earlier, that does not mean Monterey. Apple rather confusingly changed how they number macOS as of Big Sur. macOS 10.12 is actually Sierra, released in 2016. Monterey, released in 201, is officially macOS 12, and Apple does recommend APFS (or at least it's the default) for that version of macOS.

Regards.

The startup drive format for macOS 10.11 El Capitan, 10.12 Sierra, 10.13 High Sierra and 10.14 Mojave must be Mac OS Extended because those versions do not recognize APFS.

If you want to run macOS 10.15 Catalina, OS 11 Big Sur or OS 12 Monterey, then the internal startup drive MUST be formatted with APFS.

Macray1950 wrote:

...So propose to relaunch Recovery, erase HDD again, install OS [hope this will be Monterey and not OS X] and then exit into the OS. If that works I'll try and get TMachine to re-install data from the day it all stopped [24/10/24]. ...

If OS 12 Monterey is your goal, then your drive format must be APFS.

After you get your OS reinstalled, you can then use Migration Assistant to recover your user data and files from the TM backup drive. Recover all your files from a Time Machine backup - Apple Support

It would be easier to reformat the drive with Disk Utility.

Disk Utility User Guide for Mac - Apple Support

Once you reformat, it erases the drive. So if you want to reformat the drive ensure any data is copied to a safe location like another External Hard Disk prior to formatting the drive.