Not necessarily.

우리는 네가 바쁘다고 추측했어.

네가 바쁘다고 우리는 추측했어.

Both are possible and right.

Your sentence is much better. But you can use both of them.

네가 - You 내가 - I

I like to think that they are very similiar. 나 보다 = to assume 다고 추측하다 = to guess

I'm not sure though honestly