Jetbrains Rider is a lovely alternative that I use every day, but I wouldn’t call it lightweight. There’s a trial if you want to give it a shot. It has excellent integration with the Unity environment - as good or better than VS.
You can try Sublime Text (guide here: https://discussions.unity.com/t/646239 ) which is ultra lightweight and (imo) extremely well designed. It’s not going to have the kind of Unity-specific support you’d get from VS or Rider, though.