In the author’s personal view, the best substrates for Carebara Diversa are (1) Burnt Soil, and (2) ReptiSand.
Key benefit of Burnt Soil is that it is devoid of organic matter, which means that it tends to have much less mites and unwanted pests as compared to potting or volcanic soil. Also, it is fairly loose busy holds shops when reasonably hydrated, Ensuring adequate ventilation and humidity for soil dwelling species such as Carebara Diversa. Often in their natural environment, the species has been observed to dwell in clay-like substrate which is predominately comprised of burnt soil. When using burnt soil as a substrate, you must remove ant garbage frequently every few days to prevent the build up of mites and unwanted pests.
Key benefit of ReptiSand is that it, being made of quartz and other crushed mineral stones, is almost mite-free. From personal experience, ant garbage can pile up on ReptiSand for weeks without any mite infestation. However, it does not retain humidity well, and must be hydrated at least every alternate day, failing the substrate will fail to retain shape (which you will observe nest entrance & tunnel collapses, etc).
The author notes that many Carebara Diversa owners have successfully kept large colonies without any substrate, period. This is possible with proper conditioning, eg, from the onset of a founding colony. However, the author would caution that substrate-less environments should only be attempted for Carebara Diversa if the ant keeper is sufficiently confident that he can retain optimal humidity, temperature and lighting requirements.