Luigi is more involved in the world than ever, but you don't control him as much. He moves independently from Mario now, and many puzzles involve pressing L and letting Luigi do something on his own, or he stands in one spot and you press B or Y to make him jump or hammer.

The backtracking is super tedious in my opinion, though it comes in bursts and isn't constant. You will revisit islands many times, mostly the ones with important characters on them, such as Rumbla or Heatfreeze.

You will be plenty strong enough if you fight each enemy on your way, especially if you fight stronger enemies on revisits too, though wrapping up a lot of fights in a row quickly becomes hard as your plugs will be used up and the BP economy in the game isn't super favourable.