We just completed our upgrade from 7.5HF5 to 7.6HF6 - we went throught the process first in our DEV environment then completed it in our Production environment.
As for your steps we did primarily what you state below; except we restarted the NS server after the Install of 7.6.
We did push the agent upgrade to all of our 4 site servers and let them get their updated plugins then enabled the upgrade for Site Services for Task, Package, and Boot Services.
So I would agree with your steps below; but would add a restart of NS between step 2 and 3.
1. Open Symantec Installation Manager
2. You should be prompted to upgrade to Symantec Installation Manager 7.6.76
3. Reopen the SIM and click Install Hotfixes and select all of the applicable hotfixes and click begin install
4. Push the new agent to all site servers
5. Enable the "Task Service Upgrade (x64)"
6. Enable the "Windows package Server Agent Upgrade"