Im sorry to perhapps even heat things up a bit but I disagre with both

. Any particulate substratre is dangerous at this age and with this species. Bark is dangerous because our beardy can easily choke on a peace and because is totaly inadecuate. Sand is dangerous because it can cause a deadly gastrointestinal blocage(sorry varanio

) Calcisand and normal sand both have it´s hazzards. I know many people keep this lizards on sand and dont have any trouble, but why take the chance and risk your lizard´s life? It can happen ! I even posted a few months ago how a close friend of mine lost a baby beardy this way and another person lost 2 leopard geckos, all to sand

If you want to use sand, at least you sould wait until he´s at least 1,5 years old in wich age he´s going to be much less vulnerable to this. Until than use something like paper towels as substrate wich will also help whit your mite problem. I would remain suspicious about that wood chuck as being the sorce of the mites. I also think its to big for that terrarium size. If possible cut it half and put also some smod rocks,fake plants,etc. Your criter sems prety healty

You have UV ligth?Also make shure that wood isnt from any species of pine or any other coniferos tree. Oak is fine