Himmelsschmiede: so adorably tiny

[Cray]

On the other hand, a civilisation born in that era might have become despondent,

If it's a large, diverse civilization like the terragens then such universal mood swings seem unlikely. You might have individual groups get depressed that their long-term project isn't going well while the group planning to be around for the Heat Death is having a grand old time in their slow-time virches and their MPA-equivalents are having a blast building exotic mega-habitats.

We don't seem much evidence that our visible universe is, or has been, inhabited by ancient advanced aliens, but maybe we could find such evidence if we looked hard enough.

Perhaps they learned resource conservation and didn't want to burn up the universe's resources. Terragens' civilization is filled with stories of controlling resource depletion, from Earth, to gray goo outbreaks and Blights, to pink goo problems. Mostly, it has achieved some self-control.