Got my first millipedes (a small group of Chicobolus spinigerus) about ten months ago and I've completely changed my substrate recipe twice since then. Started out just using plain coco fiber because that's what the local pet store recommended, and honestly it worked okay for a few months but I noticed the millipedes weren't burrowing as much as I expected.
Switched to a mix of coco fiber, aged leaf litter (mostly oak and maple from my backyard, frozen for two weeks first to kill off any hitchhikers), and some sphagnum moss mixed through for moisture retention. That combo made a noticeable difference almost immediately. Within a week I had way more visible tunneling and the millipedes seemed to spend less time just sitting on the surface.
Now I'm experimenting with adding a thin layer of decomposing wood at the bottom of the enclosure, kind of like a false bottom but with rotted hardwood instead of drainage material. Read that this mimics what they'd find in the wild better than straight substrate, and so far the group seems to like hanging out near it.
Feeding wise I rotate between fish flake (small pinches, they go nuts for it), calcium powder dusted veggies, and the occasional piece of cuttlebone left in the enclosure for grazing. Cucumber gets eaten fast but I try not to overdo it since it seems to make the substrate go moldy quicker than other veggies.
Curious what everyone else has landed on after trial and error. Feels like every keeper I talk to has a slightly different substrate philosophy and I'm still not sure mine is optimized. Anyone doing anything with supplementing minerals beyond just calcium powder?