cut straight.

Well there's your problem! Unless you're prepared to skim coat and flatten the ceiling, you've got to scribe your trim to it (and even then the result will be "less bad," not "good"). Straight won't work!

You can't do trim on trim because it's too much ornamentation for those modernist cabinets.

This is a perfect example of what folks often don't understand about modernism: they think it should be cheap because it has simple shapes without fancy ornamentation, but they don't realize the ornamentation hides all the crimes. To do modernism right you have to have precision instead, and that actually costs more than fancy trim.

Frankly, the drywaller needs to be called back in, because he didn't understand the assignment.