Controlling humidity is a good idea. But you needed spend a lot of money.
Instead of plumbing wiring for a goldenrod unit, go ahead and pull a power cord into the safe and install a 25W incandescent bulb instead. Generally speaking it will do the same job and provide you a source of light too. If you're clever, you can set it up on a timer to regulate the safe's temperature.
I prefer using VCI products. My handguns are stored in silicone impregnated Bore Stores along with a cardboard VCI chip (from
Van Patten Industries) Other products of theirs protect the long guns just fine.
When using a safe remember that if the safe temperature is much higher or lower than the ambient room temperature, condensation can form. Especially when the cool contents of a safe are exposed to warm, humid air. The moisture condenses on the cool metal surfaces.