Top soil mixed with white sand and decomposed granite on the bottom layer and then a layer of cypress mulch on top. The bottom layer keeps super moist uplifting the humidity and cypress keeps dry so they don't get bacterial infections or shell rot.
I don't think there's a need to change it often. Originally we're only planning to change it once every 6 months or so (or if it gets really gross. Our fir bark (almost 4 months) is still in great condition. and we plan to keep it to use it for them when they are either bigger, or if we get another tort. (we are loving the sulcata but we stay in a pretty tight place with hardly any room for a tort of ginormous size)
Cypress Mulch (Is what I use). I soak mine 2x daily so Bogey just defecates and urinates in the water. I do spot check the entire enclosure daily though. Picking up food throughout the day, checking water levels, checking temps. But, I stay at home and realize not everyone can do all of this.. I check on Bogey like 20+ times a day.
I stopped using substrate after experimenting with a lot of other things, my star always develop rns, it only stopped when i removed all substrate and i only use tiles. it has something to do with our humidity that goes up to 100%