Can I safely heat 12in height tank with loose substrate?

Is it possible to use a 36x18x12 tank with loose substrate and a halogen light? I currently use a dimming halogen during the day and a ceramic heat emitter at night? Currently, I'm using slate tiles, but I’ve heard they can damage the joints over time, so I’m considering switching to a 70/30 topsoil and sand mix. However, I've heard that using loose substrate in a 12-inch tall tank could cause the halogen light to be too close to the substrate (though I'm not sure if that’s true). 

I especially want to make sure it’s safe as my gecko loves to lay out directly under the heat lamp. I can assure you that his temperatures are within the recommended range, and he has plenty of hides.

I’d like to upgrade him to a 4x2 tank eventually, but I’d prefer to make the substrate switch sooner rather than buying a 36x18x18 tank in the meantime.

Is it true that the halogen light would be too close to the substrate? I’m using 5-inch dimmable domes because of the plastic divider in my Exo-Terra tank lid, but I’ve thought about replacing the lid to allow for a light stand. However, I’m concerned about heat loss with a light stand. What’s the safest distance for the halogen light to be from the substrate? Can the 12-inch tank height be used safely with this setup? If not I intend on getting a Reptizoo 36x18x18 as Exo-terra seems not to be sold anywhere near me anymore. There are some on facebook marketplace but I'm afraid of secondhand tanks having the risk of things like crypto. Thank you for any advice you may have.