Mierin

I believe the Skybreaker progression is pretty much about each of them figuring out what Justice means to that specific Skybreaker.

The second Oath is about simply following the letter of the law because that is the simplest form of "Justice".

The third Oath is for them to select someone or something to guide them towards what is right.

The fourth is about having a personal quest and figuring out how to deal with the fact that Justice and Law don't always mix.

And the fifth Oath is probably realizing that in order for them to do what they think is Just they have to become their own Law.

Shallans thoughts during chapter 25 made me think that the Strata were Stormlight power lines of sorts, bringing Stormlight around all of Urithiru for fabrial and Radiant use while also lighting up the corridors and rooms.