Q: iMac got stuck on white screen of death

Problem:
On startup I hear the dong and the Apple logo appears.
iMac starts with loading. Stops in the middle of this, and does the iMac into a full white screen.
iMac (21.5” mid 2010 - model A1311)   runs (since an update a few months ago) on OSX El Capitan 10.11.5.

Already tried options:
All cables and 10 seconds after restart
Via Shift - Opstarsten in safe mode => After restart same problem
Through Cmd + Option + P + R - NVRAM reset
Through Cmd + R - Disk First Aid => No problems found
Through Cmd + R - Recovery backup from time machine for problem was
Through Cmd + R - OSX reinstalled
Through Cmd + R - OSX reinstalled on an external disk hade. Bar remains at half load hanging, but now is not in full white screen.

Last option now seems to erase the disk using Disk Utility fully and to proceed to a new installation.

Could this be a solution? Or are there (less drastic) options?

Thanks for the help!