After school, he joined the Golden Lions Academy and was named in the squad that participated in the 2013 Under-19 Provincial Championship. He started all twelve of their matches during the regular season, scoring five tries. These tries came in the home and away matches against both the and and their final match of the season at home to, as the Lions finished in second position on the log to qualify for the semi-finals. Marx started their 27–25 victory over in the semi-final as well as the final, where his side lost 23–35 to trans-Jukskei rivals. Marx was named in the ' squad for the 2014 Vodacom Cup, but didn't appear in any matches for the team. In the latter half of the year, he established himself in the squad that participated in the 2014 Under-21 Provincial Championship, starting all thirteen of their matches during the competition. It proved to be a very prolific season for Koch as he scored a total of eleven tries, the joint-second most in Group A of the competition. His try-scoring spell started in their second match, when he got two tries in an 82–6 victory over. After a try in their victory over in Round Five, he got a hat-trick in a 113–3 win over Border, a single try against and two tries against in consecutive matches. He got another in their penultimate match of the season against the s, as his side finished in third position to qualify for the title play-offs. The last of his tries during the competition came in their semi-final match against the BlueBulls, but it proved futile as the team from Pretoria won 23–19 to eliminate Koch's side from the competition. At the start of 2015, Marx made five appearances for in the Varsity Shield competition. He scored a try in their match against eventual champions in a 40–26 victory and scored two tries against a week later. He helped Wits finish top of the log, but didn't play in the final which UKZN won 29–24. He returned to the Under-21 side for their 2015 season, making ten starts and scoring tries against, and during the regular season as the Lions secured the final play-off spot by finishing fourth, and scored a try in a semi-final defeat for the second year in a row as won 43–20 to progress to the final at the Lions' expense.
Golden Lions / Lions
Marx made his first class debut on 8 April 2016, starting the 's opening match in the 2016 Currie Cup qualification series, a 23–27 home defeat to the. In his second match, against Namibian side the in Windhoek, Marx scored his first senior try in the 32nd minute in a 66–12 victory. Further tries followed in another defeat against a, and in victories over a and the. In July 2016, Marx was named in the starting line-up for the Super Rugby team for their final match of the regular season against the in Buenos Aires.