My understanding (poorly educated in Greek mythology) is that Sisyphus is being punished by performing a never ending task for all eternity, there’s nothing saying he can’t or isn’t allowed to enjoy it.

I don’t believe crushing shit is prohibited, as long as the shit being crushed is either on his way up, or the boulder’s way down. In this situation it seems both options are within Sisyphus’ allowed routes, and he gets/has to crush some shit either way. So yes, Sisyphus is happy either way.