Should my iMac hibernate automatically
Your iMac should automatically spin down its disks and go to sleep.

Use the following command to view a detailed log showing your iMac's power history:

pmset -g log


Look out for applications that may be keeping your Mac awake and active. Applications and other processes can keep your Mac awake if they are providing a service, such as file sharing or remote access.

Do you have an active ssh session or terminal session running? This can also be responsible for keeping your iMac awake.

Be aware that Apple does not officially support hibernation on desktop Macs. Your setting hibernatemode suggests you have this feature enabled.

Disclaimer: I am involved with a commercial product called Power Manager, it can help make your Mac sleep more predictably.