hang on startup

Hi - I can't get past the startup bar.. it just hangs - I've left it overnight - . Ive restarted - zapped p-ram - done an internet reinstall - run diskcheck - (says all OK) .. just cant get past it .. Currently I can access the iMac hard drive by starting in target mode onto my macpro so I can see the hard drives and i have everything I need off it... I have tried to reinstall from time machine but that failed too..

Is there anyway to see of the hard drive is failing ? - Disk first aid said it was OK.. I can't see what else it could be.. Its a 1tb fusion drive

2012 iMac - catalina