How to apply light settings across ALL scenes without having to set each manually?

Building on what Spiney suggests above, you can additively load scenes. When you have loaded many scenes, exactly ONE of them is the active scene. That’s the one whose lighting is used.

Here’s some more reading:

Additive scene loading is one possible solution:

https://discussions.unity.com/t/820920/2
https://discussions.unity.com/t/820920/4

https://discussions.unity.com/t/824447/2

A multi-scene loader thingy:

My typical Scene Loader:

Other notes on additive scene loading:

https://discussions.unity.com/t/805654/2

Timing of scene loading:

https://discussions.unity.com/t/813922/2

Also, if something exists only in one scene, DO NOT MAKE A PREFAB out of it. It’s a waste of time and needlessly splits your work between two files, the prefab and the scene, leading to many possible errors and edge cases.

Two similar examples of checking if everything is ready to go:

https://discussions.unity.com/t/840487/10

https://discussions.unity.com/t/851480/4