If someone would for example normally skip a Sunnah prayer out of tiredness, but forces himself to pray it so it is more likely Allah blesses them with something they want (such as a righteous wife or good health of a child), is that acceptable or does it ruin Ihklas?

It does not ruin your ikhlāṣ if you pray it for Allah and also hope He grants you something good in this world, and Shaykh Ibn Uthaymin said there is nothing wrong with wanting the good of both this world and the next. What is forbidden is praying only for the worldly thing.