How to replicate a UObject property inside a struct?

Hey, I just solved almost the same problem as you are attempting to!

I think you can mark the InventoryStruct, BackpackStruct and slotList as replicated
then in your ActorComponent’s ReplicateSubobjects()

// make sure to register your replicate vars in GetLifetimeReplicatedProps

bool UYourActorComponent::ReplicateSubobjects(UActorChannel* Channel, FOutBunch* Bunch, FReplicationFlags* RepFlags)
{
	bool bWroteSomething = Super::ReplicateSubobjects(Channel, Bunch, RepFlags);

    // this replicates the TArray
	bWroteSomething |= Channel->ReplicateSubobjectList(InventoryStruct.BackpackStruct.slotList, *Bunch, *RepFlags);

    // this replicates the actual TArray content
	for (auto* Slot : slotList)
	{
		bWroteSomething |= Channel->ReplicateSubobject(Slot, *Bunch, *RepFlags);
	}

	return bWroteSomething;
}

The code might have some syntax error or something since I am writing it top of my head.

Here is another good link :

Hope it helps!