I’ve completely layered the inside with 1/2” fire board. I was planning to do a second layer but my vault isn’t very big and it’s really cutting into my space. How much of a temperature difference does a second layer make in a fire?

Could you put the layer on the outside of the safe?  Perhaps both layers.

I have a layer on the outside. It’s welded steel. Before I fit it to the room, I cut all the drywall out. Filled it with foam and fire board. There is one layer of fire board inside. The only reason I’m thinking of going with another layer is because my house is basically a barn. It’s sided with barn wood. I live out in the woods. If there were a fire, I’d say 30-45 minute response once someone noticed.

If you were particularly concerned you could paint the exterior (final coat) with intumescent paint.

I am not sure how well board on the inside will fare in a fire.

I would buy Ring (or similar) monitored smoke/CO detectors. They aren't very expensive.