A president is indeed most influential with Congress when he's new in office, which is when legislators are most apt to give him, his team and his ideas some political leeway. But that first 100 days also happens to be when a president is the least likely to know just what to do. No matter how smart a new president is on his first day in office, he always gets better, and so does his legislative craftsmanship. Achievement almost never emerges from proposals introduced, let alone enacted, during the first 100 days of a new administration. The vast majority of major laws enacted since World War II were introduced well after inauguration day, often making their first appearance years ahead of final passage.
Great achievements involve perseverance over time.
Presidents usually make their greatest mark by building upon past successes. That is how the air and water became cleaner, communism was contained, veterans of war were helped, and diseases were conquered. That is also how past gains such as ensuring safe food and drinking water will be protected, and past disappointments such as the failure to renew poor communities will be reversed. Achievement rarely involves a single breakthrough law such as Medicare or the Americans with Disabilities Act. Once launched, government's greatest hits involve repeated efforts to fine-tune, expand and repair.