Possibly, the best solution is to put your editor-only actors in a sublevel (one that’s streamed in and exists in the same space as your main level). Then, when packaging, you’d have to exclude that level (either remove it from disk for packagin or only load it on WITH_EDITOR).
I’m not sure two levels is a great workflow. Using Layers would be better, but I don’t know how you’d turn it off for packaging (aside from deleting that layer).
I think the Epic approach is to find an alternative to many actors. The navmesh bounds is a single large actor that aggregates the navigable area within the volume. If you want designers to place your actors and adjust properties on them, then you might have to do a lot of custom work (so actors is a much easier approach).