I totally understand your reasoning. I have had a gecko die of impaction as well., it was a young gecko I purchased from a petco. She came to me that way.
i am very cautious with them. Slate is a good substrate, i personally do not like Repti Carpet I have seen problems with that in the past as well.
I have seen a soil, sand mixture that someone used with live plants. it looked really nice. After having geckos after awhile, I found they like to dig around in their substrate. Some have made their own hides underneath a piece of drift wood in the tank.
I give mine a container with vermiculite and moss (moist hide), they can dig and lay eggs in there and I don't have as high of a risk of impaction. I'd just recommend a non-loose substrate until the gecko is over a year, then you can change if you are set on it, thus not risking the baby gecko.
Just my suggestion. I care too much for my geckos now to take the risk.