Good move.
Yes the game is well designed. But dont feel you have to rush it. Enjoy the game solo for as long as its enjoyable that way.
And DEFINITELY explore all of the nations. They are unique with different advantages and disadvantages. Some are built around specific modes of play. Offensive, defensive, research, horde, stealth, etc. Find the nation which best suits your style of play.
When you enter into MP you will feel that you have discovered the one obviously overpowered nation. But keep in mind that everyone else has also, and theirs is not the same as yours. Yours is "obviously overpowered" in solo play only because it works great with you as its master. Then the fun begins all over again.
Gandalf Parker