27" iMac Retina 5K Late 2014 Fusion Drive problem

Hi my 27" iMac Retina 5K Late 2014, 3TB fusion drive appears to have a Fusion Drive problem. It won't boot up. When I power up I get the Apple "Bong", the Apple splash screen and the progress bar appears but the progress bar stalls about one third of the way through and the iMac hangs.

I have tried starting up in Safe Mode and in Recovery Mode and sometimes I can run Disk Utility which shows the drives. When I run Disk First Aid no issues appear to be found with either the SSD or the HDD. Sometimes I can start up in Safe or Recovery mode but most times I get the Startup Progress bar and the iMac hangs.

I have reinstalled MacOS Big Sur via Internet Recovery but the problem still remains.

Am I right in thinking that the SSD part of the Fusion Drive stores the MacOS and that it is the SSD that is more likely to have failed? Or would my issue likely to be with the HDD or perhaps both SSD and HDD?

Thanks in anticipation,

Tom