What the next step would be. Hard to say, I started to think about the game "Monument Valley" that is also heavily on visuals and simple puzzles. I don't really remember the gameplay on said game when it comes to "next level select" and such, but believe that the character exit the door (level done) and then comes out from a new door to the next level without any extra menu or so.
Your version could be that you simple automatically gets to the next level once you hit the "portal" in the end, so it seem more seamless and avoid needing to click "next level" since that is usually what the player wants anyway.
The levels can also be bigger and having checkpoints and you progress downwards.
Monument Valley also works with several different color schema so that could be an idea to progress with different background colors and such.
I am not a fan of the "zone X" and "level X", feels very mobile, but all depends if that is what you want. I randomly thought about "Baba is You" but think the level select might not match this. Trying to think of something more fun.