In your case, it might be better to simply bite the bullet and cope with the overhead. Its unacceptable levels of overhead in a general toolkit, but it should be fine if it'll save you (possibly significant amounts of) time.
Saving render data might not be the best idea - the only way to save it properly is in a scene, and you'll end up with many many scenes if you did it this way. This might not be a bad thing if you write an editor script to automatically update all tilemap scenes from tmx though...
Edit: One more thing - you can't access tilemap info from the render data. You need the tilemap object for that. Scenes might be the best choice here.