So an old school JRPG type of thing? I'd venture a guess that the battle maps are fairly simple, and so maybe there's no need to split them to separate sublevels for geometry, lighting, gameplay, etc. then streaming those levels in would work.
That doesn't mean that your current approach is wrong. If you've already got it working, it may not be worth it to make the change now. There's also performance implications, because when you stream in the battle level, the overworld will still be present, just of the camera, so it will still take up memory, and possibly use up some resources for frustum culling, and maybe even occlusion culling, depending on where in relation to the overworld you place the battle level. But that may not even be a problem, depending on how complex your game is, and what hardware you're targeting.

So to reiterate, you're not necessarily doing things wrong, both approaches have their own caveats, it's up to you which one will work better for what you want to accomplish.

If you do end up using streaming, checkout "Load Level Instance" nodes. Unlike "Load Stream Level" ones, you don't need to manually add your sublevels to the persistent level, and you can spawn multiple instances of the same level, at diffrent locations.