I am using a small vacuum desiccator to thoroughly dry DMT crystals and changa.

I use calcium chloride as a desiccant, which is a common broad-range drying agent.

However we remove mostly naphtha, heptane or alcohol from the products, not water. This makes me think whether calcium chloride or other salt is suitable for a dessicator.

Anyway, it works very well and the crystals and changa a very dry when leaving in a desiccator overnight @ -400 mmHg.

What do you think about removing traces of solvent from DMT and changa, the solvent being usual hydrocarbon solvents and alcohols?

I think it's an excellent idea to remove all solvent from the product. Considering that you smoke the stuff.
Sadly not many people have a strong vacuum available to do so, including me.

No common desiccant will help you remove hydrocarbon solvents. Best way is to finely divide the product and put under the strongest vacuum available. How long does it take to remove it all? Highly dependant on solvent, product, and vacuum strength.
The usual process for me, once upon a time, was powderise, place under 0.1mmHg for 24h, then check by nmr.
Do the best you can do. If you have a vacuum pump, you're lucky, use it.

My strategy is to remove as much water from freebase as possible by placing in small paper envelopes within MgSO4 container for a week and then FASA. Result is air-dried 12h, then crushed and oven-dried at 80C for 3-4 hours, crushed as much as possible every 30 min. Yes, for fumarate of course
Acetone traces (if any at all) are much more preferrable than any other np