I understand 大臣 means "minister" and 閣僚 means "cabinet minister", but I am curious what's the difference between these two terms ?
Both mean the same thing. The difference is how they combine with other words.
According to this, 大臣 is more about individuals while 閣僚 is more about the ministers as a group of people.
My impression is that they are not so interchangeable.
- 外務大臣 Minister of Foreign Affairs. Not 外務閣僚
- 末は博士か大臣か will become a PhD or a minister in future - an oldish set phrase to describe a promising child (although I don't see much future for Japanese PhD's and ministers now). 閣僚 is not possible.
- 首相は関係閣僚に対応を指示した Prime minister told ministers in charge to take necessary actions. 大臣 is not possible.
- 閣僚経験者 A person who was a minister in the past. To me 大臣経験者 sounds OK, but it is much less common.