I've had to copy this 'mdadm.conf' file taken from old OMV3 to the current OMV6
No, if the array is recognised in OMV6 then it should be possible to create an mdadm conf in OMV6 from the command line
I'll try and clarify what I said #19
You have OMV6 working, shutdown the server and connect your raid drives, mdadm (which loads during OMV's boot process) should detect those and begin rebuilding the array.
Once the server is running login to OMV's GUI and under Raid Management the array should show as rebuilding
That is all you have to do at this moment, if this works at least we know that OMV6 has recognised the drives and has begun reassembling the array.
Likewise, if you get nothing displaying in Raid Management then we have to approach this differently