I’ve been thinking about this “spouse being best option for קנה לך חבר” opinion and I’ve decided that I don’t agree with it. Ever. Not that your spouse should not be your friend, but that it doesn’t work in the context of the Mishna.
First, to prove technically that that is not what the Mishnah means, chaver does not mean friend, despite the Yiddish and therefor Hebrew usage of the word. It means something closer to the English word peer. A spouse cannot be a peer because of the difference in gender.
But more importantly, there are vast differences between the spiritual issues that males and females face, and asking someone of the other gender to fully understand one’s issues, to empathize and then to give help is not really possible. Being a chaver, in the sense of the Mishnah, means someone who can understand you fully, and a spouse, not matter how close the relationship, cannot.