Actors already have the concept of Tags which can be specified in the property details panel. There is a HasTag function and if you want to check if a given Actor has a tag. You could wrap that up with Get All Actors of Class inside a macro if you wanted to make a find by tag function in blueprints.
Streaming in a subset of actors in a level is not something that is reasonable to do. The relationship between a level and its actors is not conducive to loading partial sets in different batches. If you wanted a behavior similar to what you’re describing I would probably look at layering a few streaming levels with each set of actors you’d want to bring in independent chunks and stream each level in at the point you were thinking you’d specify the tagged set of actors to be loaded.