Dehumidifiers work well. I know some guys with guns that are worth more than my house and they use some of the battery powered dehumidifiers (they put more than one in each safe, but the safes are not in a climate controlled area) They work well and guns have ZERO rust. Inside the house where my safe is, I use nada.I don't worry about humidity in an air-conditioned or heated space.
Keep your firearms well-oiled, and periodically open the safe to let fresh air in and stale air out.
I tend to think dehumidifiers are a gimmick.
20+ years in and out of the safe, and no rust.
Be well.