Heat lamps naturally lower humidity, and without a substrate to hold humidity it will be difficult.
Once mites are dealt with a mix of coconut fibre and moss will bring the humidity up nicely.
In terms of the mite treatment there’s a product you can get which I have used numerous time but do your own research it’s a mite that hunts snake mites called Taurrus.
These will rapidly hunt and consume the mites then die off and are completely harmless to your reptile.
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