Well, having it bolted to the foundation doesn't mean that the metal of the safe must ride right on the foundation itself, unprotected.
One method: climate control, of both the air and the floor. Some folks have floors with built-in heating. That would certainly cut almost all the temperature change from infecting the safe's structure.
You could also build your own heated "floor" out of a section of thick, high-grade marine plywood and the appropriate "heating" coils snaked through its guts.
Most folks, though, are going to simply have a cement slab, which will go through the full swings in temperature that the outdoors does.
You might consider using a barrier that can help block much of the temperature swing transmitting the result directly to the steel of the safe. For example, imagine sealing that 4x4' section of concrete, using a 1" sheet of high-quality, marine plywood that's been epoxied "shut," an then topped with a couple sheets of high-grade moisture barrier (plastic) that's used in construction. Placing the safe on this and then bolting it to the foundation can create a decent buffer between the harsh concrete and resulting temperature changes. Won't stop it, but it'll buffer the worst of it from the metal of the safe. If you've got a serious temperature problem, you could even make your own heated plywood "floor" for the safe, much like folks install in homes. Though, I couldn't vouch for the result, given usage in a room without other climate control.
You could acquire one or two of the "thinner" safes (front to back), for storage in closets. Of course, that'll cut your closet space by quite a bit. Still, that'll bring the units inside to a space that has better temp controls. If you've got a big room in the house, you could even build in a 3ft deep false wall, for that extra "closet" space.
What I do is predicated on the relatively small temperature swings I have to contend with. It gets cold, here, sure. But, it won't swing from 25*F at night to 80*F during the day, then back again. Very rarely. And, in my home, the concrete is not changing temps very quickly, given the nature of the build. Generally, it stays within a fairly tight temperature range, which cuts down on the moisture production pretty well. I've not noticed any condensation on the outside or inside. I've got a long goldenrod @ 200cuft capacity, a can of silicon gel pellets @ 50cuft capacity, and one of the EVA units @ 30cuft capacity. I recharge the can and the EVA monthly, and they needed it. So, yes, the temp swings are boosting the capture of moisture, but I also am aggressive to protect that ~18cuft interior space with (claimed) 250+ cu.ft. moisture-capturing capacity from the goldenrod + can + EVA. Still, it requires frequent recharging.