In 2015, Buchmann denied the strong sprinters their chance at the German National Road Race Championships when he attacked from a leading group to take the title. During the subsequent Tour de France, he finished a strong third on the stage finishing at the top of the Col du Tourmalet.
2017
After a quieter 2016 season, he finished tenth at the 2017 Tour de Romandie, losing a podium position only during the race's final stagetime trial. Buchmann then finished seventh overall and first in the young riders' classification at the Critérium du Dauphiné, one of the major preparation races for the Tour de France. During the last stage, he finished ahead of strong general classification riders such as Chris Froome, Alejandro Valverde and Alberto Contador at Plateau de Solaison. Buchmann commented after the stage that he was delighted to have been able to "stay with the best" of the race. At the German National Road Race Championships the week after, Buchmann set up the victory for his teammate Marcus Burghardt, finishing in second place. His form faded slightly at the Tour de France, where he was set to work for team leaderRafał Majka, who withdrew early; he eventually finished in 15th place overall.
2018
Buchmann opened the season strong, with a 10th place overall at the Abu Dhabi Tour. He followed that up with his best result, at that time, 4th at the Tour of the Basque Country. He finished 9th at the Tour de Romandie, and 18th at the Eschborn–Frankfurt race. At the Critérium du Dauphiné in June, Buchmann finished sixth in the general classification, repeating his strong performance from the previous year. He attacked multiple times on the final stage, but was brought back and finished the stage fourth. He was selected to ride the Vuelta a España. He performed well in the opening stages. On stage 4, he attacked the group of favourites on the finishing climb of Alfacar, gaining time which moved him into second place overall, within seven seconds of the race lead. After falling back during the subsequent mountain stages, Buchmann eventually finished the Vuelta in 12th place overall, his best result in a Grand Tour up to that point.
2019
Buchmann raced the Tour of the Basque Country, winning stage 5 after attacking on the penultimate climb of the day to take the overall lead. He lost the leader's jersey on the final stage of the race. In the closing metres, Buchmann took a wrong turn and lost significant time. The time he lost through this was later reinstated, meaning that he finished the race in third overall. At the Tour de Romandie, Buchmann finished seventh overall. In preparation for the upcoming Tour de France, Buchmann raced the Critérium du Dauphiné, where he finished third overall, after Adam Yates abandoned the event during the final stage, handing the podium spot to Buchmann. He remained in good form all the way through the Tour, answering nearly every attack by the elite general classification riders. By the time the race reached the Champs-Élysées, he was barely two minutes behind race leader Egan Bernal, just off the podium in fourth place.
2020
Buchmann started his 2020 season strongly, winning the Trofeo de Tramuntana, the second leg of the Challenge Mallorca. He was part of a breakaway that formed on the penultimate climb of the day and used the descent to create a gap, which he held to the finish. His last race before the season was interrupted by the COVID-19 pandemic was the UAE Tour, which he was forced to abandon after a crash. He is set to compete in the Critérium du Dauphiné before leading his team at the Tour de France.
