Millipede enclosure setup (advice welcome!)

[bored9x]

Hello, all. I'm a first time millipede keeper and I'm overly paranoid about all of the things that could be wrong. No pedes yet, but I plan to get some at a reptile show next month, with ivory, giant american, and smoky oaks top on my list (although I would happily house smaller or less surface active millies if they would live better in this tank). No plans of breeding as I know space would probably not allow for it (maybe someday if I upgrade!). I would love if folks would take a look at my enclosure and let me know what they think of it.
Tank: "Thrive" (PetSmart) brand critter keeper in size "Large" with lots of lid ventilation. Roughly 3 gallons.
Soil mix:
- 1 part organic topsoil (fertilizer and pest/herb-icide free)
- 1 part old oyster mushroom grow mulch that has been broken down to basically sawdust
- handful of shredded oak leaves (cleaned and frozen overnight)
- Temperate white springtails from SmugBug have been introduced, although I haven't seen them since I did so.
Top layer mix:
- More oak leaves (although I plan to add maple and sweetgum once they start falling more here)
- oak sticks (some with acorns still attached, all of which were also cleaned and frozen)
- oak bark with lichen growth (frozen 24 hours and left out for a day to thaw and dry)



My questions for all of you:
- Is this a good soil mix? I know people recommend rotting wood for pedes, and I already happened to have that mulch lying around. I don't see anyone else using it, though.
- Is there anything else I should add? I plan on getting a water dish and thermo/hygrometer soon, but is there any other fixture I should look into?
- How many pedes would fit comfortably in a container this size? From what I've read, it seems like around two adult millipedes could live comfortably in a ~3 gallon tank this size and soil depth without constant substrate changes, but what do those of you with more experience than me think? I really want to give them the best possible life I can without having to change the soil out every week.
Thank you all in advance!

 

[Elytra and Antenna]

Substrate looks a bit dry and the particles too large but it might work. That much ventilation is a death sentence.

 

[bored9x]

Ah, I figured as much about the ventilation. I went ahead and covered the top with some plastic sheeting. I also added some hygrometers and a water dish (two might be overkill, but... better safe than sorry). Another millipede owner I spoke to encouraged me to add more leaf litter, so I did, and at your suggestion I will be breaking it down to smaller pieces. I was also told to add sphagnum moss to the wet side for moisture retention and did so as well (I forgot to mention in my original post that the topsoil mixture contains sphagnum). That's the good thing about setting this tank up several weeks in advance! Plenty of time for troubleshooting!

 

[Elytra and Antenna]

The leaves look fresh, not decayed, that can lead to mite problems.

 

[bored9x]

These were collected on the forest floor and frozen (will bake in the future but my oven is currently broken). I will be on the lookout for any mites and use more decayed leaves in the future. I've seen people recommend predatory mites for taking care of hitchhikers (at the sacrifice of springtails, which would suck since they're getting established), but I'm not sure how effective those would be or if they would hurt any millipedes. If I spot anything uninvited, I might do that.