This is a luck-based achievement and you're not the only one if you struggling on this.
In my case, i have completed all objectives in one season - and no achievement. But the achievement tracker stucks at 93%. I think, the tracker has a problem, when you complete two objectives at once. This happens at the end of the season after finishing Spain.
Be sure, that you keep an eye on the tire-related objectives. These can be done very quickly. Objectives like "Rest within six weeks" or something like that, are time wasting. But remember, you can change an objective only once.
Now some tips:
If you have forget to change the tires, you can quit the game to the dashboard. After loading your safegame you're back in the service area.
The German Rally is the hardest event in the calendar, even on easy. The weather conditions are very frustrating. Driving with dry tires in wet conditions are really bad. But driving with rain tires on dry streets are better. My experience was, if you equipped dry tires, the following stage was rainy. If you equipped rain tires, the following stage was dry. And yes, its annoying.
Hope this little guide helps.
Thaughts are welcome.