The higher aspect ratio is a result of wanting to cruise closer to the minimum of the drag curve.
In a typical light aircraft low-level cruise, induced drag makes up only a small proportion of the total drag. However at high altitudes, you operate close to the minimum drag speed -- and in particular, at the ceiling of the aircraft you operate at the minimum drag speed, where the induced drag is 50% of the total drag. Increasing the aspect ratio will generally lower the induced drag at the expense of parasite drag.
Thickness is a bit more complex. There's usually an optimum thickness for minimising drag, but that depends on the lift coefficient. From looking through Abbott and von Doenhoff, it does look as if thinner aerofoils are less draggy at higher lift coefficient (i.e. close to the minimum drag speed), while thicker ones tend to work better at lower lift coefficient.
Have you worked out the lift coefficient of your TB20 in high altitude cruise? You only need the mass, wing area and IAS.