Important to remember, they won't draw the foundation unless there is a flow on. The bees will create the space they need, as they need it, but if there is nothing to store, they just "hang out" as you mentioned. The excluder is also an inhibiting factor for them. .
I do not use excluder, or I use it in special purpose.
One reason why they do not build foundation it that they may think swarming. When they wait swarming, they do not build new combs.
If you have 2 box bees, it is not wise to restrict brooding. Good queen needs 1,5 box for brood. When it is too tight they are going to swarm.
I is summer now in Finland, 20-24C warm at day. Still I use 15 W terrarium heating in my 1 box nucs. They lay eggs 3 times more than in natural way. It takes 2 weeks and Langstroth box is full of larvas.