Whats the next step after commercial fusion reactors?

The answer is still fusion, but with different fuels. The easiest fusion reaction to do, and the one current research focuses on, is deuterium + tritium. But tritium has a 12.5 year half life and doesn't occur naturally. It has to be transmuted from lithium inside the power plant, right next to the reactor. This is a huge added complexity. The D+T fuel also emits high energy neutrons, which irradiate the surrounding materials and require shielding. You could pick a harder fusion reaction like hydrogen plus boron. Then the reactor itself has to be way higher tech, but things like fuel and shielding get a lot easier. There's also helium3 + deuterium to look into.

If you want to get really stupidly exotic, try to steal rotational energy from a rapidly spinning black hole, which is like the one thing you can take from it.

Fusion isn't likely to be a very efficient way to harvest electrical energy in the physics sense (efficiency = energy harvested per energy consumed) even today. It is so popular an idea because it is a promise of a cheap and limitless energy production, not an efficient production.

If what you are after is efficiency, hydropower plants are already close to 90% efficiency today, much better than fusion is hoped to be. If you can make the station so big that you have lots of water, gravity and water cycle powered by Sun in it, then you can put a hydroelectric power plant there.