I think that the thalamus is as closely related to the basal ganglia as it is to the cortex. The basal ganglia uses reinforcement learning to predict when the animal will receive rewards or penalties. Animals use the basal ganglia’s predictions to attempt to maximize their cumulative rewards, which drives behaviour. The basal ganglia however does not directly connect to the muscles which control behaviours, instead the cortex connects to the muscles. The thalamus is the major pathway from the basal ganglia to the cortex, and therefore is at the interface between unsupervised and reinforcement learning.
I hypothesize that the function of the thalamus is to control the cortex, with the goal of maximizing the animals cumulative rewards.