I’m not sure if this changes the question, but it’s an interesting idea.
The Rambam says that pras is a gratuity bestowed upon a person to whom nothing is owed; it is given out of sheer generosity, as an act of kindness and grace.
The Rambam says this is the difference between sachar ??? and pras – sachar is payment, and is deserved, and pras is a reward, a gratuity.
So note, pirkie avos is not saying there is anything wrong with serving Hashem for schar (according to the Rambam)