Tempered chocolate thickens
Based on what you’ve written, i can think of two possible reasons why you’re getting thick tempered chocolate.


As the ratio of cocoa solids increase, the viscosity of your melted chocolate will increase. If your chocolate is high on solid content, you might need a bit more cocoa butter to decrease viscosity.
Even though, you have a water-free heating device; due to the moisture of the environment and condensation caused by cold could introduce some water into your chocolate. Adding something to bind the water might help, the go-to emulsifier to bind some water is Lecithin. 0.5% to 1% Lecithin could help with binding the water, thus reducing the viscosity in a couple of minutes.