In Metal Gear Solid 5: The Phantom Pain, is there a way to avoid or not do the [EXTREME] missions? I did all of those up until Metallic Archaea.
They are completely optional unless you want 100%, I got the true ending without doing any of them. You only need to do the missions and side ops that are marked in yellow. When you run out of those, just 'pass time' by doing any side ops or replaying old missions for a while, and eventually you'll get calls progressing the story and unlocking more yellow missions. You'll have to do that a couple of times.
You need to do the extreme missions, however, I found out that there are ways you can do them without being an active participant. Like in mission 31 where we have to take Sahelanthropous, i hid behind cover in a valley where he couldn't see me at all and order bombardments and air support from my i-droid. I hope i was of some help without spoiling it for you. This is the best part of the game that it stays true to the idea that you can choose anyway to approach the mission.