It would be helpful to get a temperature reading on the glass under the substrate right over heat mat. The heat mat thermostat probably needs set just a bit higher to achieve 90F inside the enclosure under the substrate. They will burrow under substrate, I wouldn't fuss about it. I can't tell if you have 3 hides in there or those rocks are just decoration. There should be at least 2 hides, and the rock on the right looks like its probably way too big to function well as a hide for a 100g snake so you may want to think about getting a smaller one for that side for now. Otherwise things sound great.