Failed start up - 100% need erase hard drive?

I have been taken through safe start up (shift key) and reboot(?)/disk utility start up (alt key) over the phone with Apple staff.

On second attempt, safe start still didn't work.

On first attempt, disk utility said I needed to erase the hard drive and re-install (I have a time machine back up).

On second attempt, disk utility said my hard drive was fine. I tried restarting after this , but the original problem re-occured : an endless loading circle, never leaving the gray screen and loading symbol on start up.

I am happy to take it in to a genius bar , but don't need to if I 100% need to delete and re-install, as I can do this.

Have I missed something to check? Do i need a genuis bar?

Thanks!