How do we reconcile the fact that the reward of a worker in the End of Times is equal to the reward of fifty Companions, with the superiority of the Companions and the fact that a single "mud" (a measure) donated by one of them is not equal to spending an amount of gold the size of Mount Uhud?

The increased reward for some later generations does not indicate their superiority over the Companions, as reward is relative to the equivalence of the action. As for the virtue of witnessing the Prophet, peace and blessings be upon him, no one can equal it. Later generations may have great good deeds, but that does not mean they are better than the Companions. What the Companions did in terms of faith, struggle, and loyalty to the Prophet, peace and blessings be upon him, in difficult circumstances, cannot be replicated by anyone. The hadith "My Ummah is like rain" means that among the later generations there are those who approach the former ones, not that they are better than them. As for the hadith mentioned in the question, "The best of centuries is my century," it has no origin. The correct wording is "The best of people are my century" or "The best of my Ummah are my century."