As soon as you get your Suit, you'll find a portal leading back to the "normal dimension", but you'll not be able to open the doors without Power Bombs. Furthermore, you'll still take damage in the Phazon Dimension. At this point though, you have a number of options available for you to explore, and different directions you can go. And plenty of Metroids to kill.
I love that feeling of "Okay, you have all your stuff back, and you've uncovered how this game works. Now the rest is open to you. Have fun!" in games. Even if the first half of the game so far has been fairly linear, that open feeling feels earned.