此業務發展已經飽和, how to write in English???

Actually, I think the Chinese sentence is somewhat strange.
一個業務(business)可以到達頂峰.
一個行業(industry)可以在一個社會變得飽和,容立不到其他競爭者.

一個業務發展已經飽和就有點古怪了.

So I suggest you to write like this:
[The business has reached the climax.]
or
[The line of business has become saturated.]

this business is already saturated

saturated 已是飽和
over 一字多餘

this business is already over saturated