20k is not a good bonus for this company. That's the bonus plan for a run of the mill engineer just starting out there. You'll find college grads hired right out of school with a higher bonus plan.
Why? I think this is something extra on top of their usual bonus plan.
If that is the case, it is not that bad. You're probably talking going from 280k TC to 300k TC at that point.
I would assume so. It's Microsoft after all. All big companies have some kind of "bonus" in your salary especially in America. I think 20k is too low to be that. I get more than that and I work in the EU.
The “normal” (meaning you met expectations but didn’t wildly exceed) bonus at Microsoft for a L65/L66 (Principal) level is 20% of base salary. Officially the range is 0-40% but the vast majority will get 20%. Base salary at this range is about $200-$250K, so a normal bonus would be $40-$50K at that level.