Does intense striving to earn money with the intention of fulfilling needs contradict the heart's reliance on God, or not? And how can one balance between the two in a way that pleases God?

Seeking sustenance is a religiously commanded act, and it is a matter of taking means, which is required by reason, without complete reliance upon them, which would undermine monotheism. The Muslim strives and exerts effort in earning, with his heart attached to Allah, knowing that whatever befalls him is what Allah has decreed for him. This principle encompasses all affairs of the servant, so he takes means in every matter. However, seeking sustenance should not be his greatest concern and distract him from the Hereafter.