Achieving proper temperature gradient thru several inches of substrate? (TL;DR)

[Armiyana]

I'm wondering if she just likes the shape of the hide on the cool side. She sometimes picks a different hang-out spot after I've cleaned and re-arranged the tank. I might try swapping it around later and see how she responds.

This could also be the case! if they are different that might be why she's avoiding it. They can be really picky at times. Maybe the warm hide doesn't feel as secure. They tend to like tight fits that are well hidden more than open spaces.
If the temps are correct after all, then I would try to figure out why she isn't comfortable there. On some of my fastest growers I switch hides out so much cause they just start outgrowing them so fast but anything too large and they get fussy. lol

I'm glad you put the little feet on! a lot of reptile branded terrariums now come with little open plastic rims on the bottom so you don't always have to, but better to be safe then sorry. Especially if it's just a regular flat base one.
When you get the heat gun, I would make sure to temp the glass as well as the top of the substrate. Just in case she ever tries to burrow down to the bottom (it happens alot with my girls) you don't want it being too hot there as well.