You are asking to change C++ constructor params without running the constructor and the only way to do this, is running a function after your constructor, which you can define yourself. Luckily, Blueprints already have such a function and the ExposeOnSpawn was neatly integrated with this function = ConstructionScript.
You got a valid question why Epic didn’t refactor this function into 1 line of code where you specify the functionname in the parameter list of SpawnActorDeferred and if I had to guess, they don’t bother and think people use these kinds of features in BPs anyway or they didn’t want to implement a function in AActor called ConstructionRound2 that only calls FinishSpawn and needs to be overridden. Probably not that easy, something is probably missing. But then again, I don’t want to know how exactly Actors are spawned, when they locate memory and what else is happening when Spawning. Maybe there is something magical I miss, but I doubt you will find such an answer here on Answerhub. It’s okay to ask though, there is always this slight chance an engine programmer has a happy day :).