Can an Integrated Circuit ROM ever be secured
You must identify your expected adversary and describe their capabilities. If your adversary is the intelligence services of some reasonable country then none of these steps will protect your ROM. Imagine someone has access to a complete physical analysis lab, including a scanning electron microscope. Security through obscurity is no security at all.

Using a different personalization for the ROM in different fab runs is not a manufacturing problem, it's just a matter of using a different mask for the contact layer. However, it is expensive and someone needs to be careful to use the correct mask for each run.