Actor Soft Object Reference (Object Path).
In the save file, you need to store the path of the object that we have collected. For actors that are initially on the level, this is a reliable identifier that does not change when the level is reloaded.
After loading the level, you need to check which of the collected items are on the level (ActorSoftObjectReference->Resolve->IsValid) and remove them from the level.
With dynamic objects, things are more complicated.
It is necessary to create your own system of reliable identifiers that will be assigned to unique spawning objects in a particular case.
The standard Unreal ID system cannot provide this.
References for the base class (actor) are cheap.