In B2B services space, under promise and over deliver do not work well.
Just let the client know what they should expect and deliver that on time and agreed quality.
And they will stick with you.
In a long-term retainer, create moments of delight by doing something the client did not expect.
But don’t do it so often that the client starts expecting it.
Because to differentiate and compete, you must put all your cards on the table.
Or else you risk losing in the face of competition.