Your steps towards making it a better terrarium sounds good. Sphagnum retain moist freakishly well and the mister will be a good way to add humidity frequently (combined with a timer, it's your best friend). Live plants keeps the humidity levels more stable as well. I do believe 60F is too cold for them to experience at any length of time. You might want to consider getting additional heating devices, so you can use just enough for every situation (both at winter, 1 at spring, 0 at winter, for example). Placement of the UTH is important as well. Do you have it on one of the sides, or beneath the tank?
UTH below the tank is a good way to create a vaporization-drainage, but it does create very little heat to the actual space in the tank. An UTH on the side, will heat the air within the terrarium and serve its purpose more efficiently.
Another suggestion would be live plants. Lots of live plants. Not only for the humidity stability, but for the hiding and climbing opportunities it provides. Of course, you might already have plenty of these, but since you did not mention it I figured I might as well say it
