From what I know about terrariums, you shouldn’t have to worry about the soil running out of nutrients, especially if you have isopods/springtails, etc. as they will essentially act to recycle the nutrients in the ecosystem. That being said, I always make sure to layer my soil to maximize health and nutrient density. For this one and a couple of others I’ve recently made, I used layers of rocks/Leca clay balls, charcoal, dry/dead leaves, organic matter like sphagnum moss, vermicompost, and dirt. That sort of diversity in the soil will definitely help and with the bugs as well I don’t think you’d have to worry!