This is the problem: tying OKRs to bonuses. It sounds so logical from a naive standpoint and yet it has so many detrimental side-effects.
Where I'm at, we have a hybrid system that works quite well. OKRs are divorced from bonuses; and they are always "Stretch OKRs", ie. ambitious ones. Essentially they are the compass where team is headed. Then there are MBOs which are tied to bonuses. These are very conservative, so that the bonuses are attainable.
With this system, in practice, the MBOs are the subset of the Key Results that actually seem achievable within the quarter.
Of course there's a functional way to play this game and a dysfunctional one.
Working, functional version: spend time defining actual Objectives, regardless of whether we know how to measure them. That last point is very important. Only when the Objectives are defined (and they are almost always qualitative in nature), THEN come up with Key Results, to try to quantify the objective (and be open to adusting that). Finally, pick out the KRs that can be realistic, and make them the MBOs.
Dysfuntional version: start at the opposite side. Define MBOs. Then Key Results. And finally, try to BS some form of Objectives to make it all sound coherent. This biases towards both what is easy to measure and easy to quantify, to the detriment of business needs.