Do you have to do the [EXTREME] missions?
Solution 1:
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.
![image of my mission list with the true ending unlocked but none of the [EXTREME]s done.](https://i.stack.imgur.com/ozK6i.jpg)