[6] In the semifinals of the men's 200 metre individual medley event of the 2020 European Aquatics Championships, Kós set a new World Junior Record with a time of 1:56.99.

[9] At the 2021 European Short Course Championships, conducted at the Palace of Water Sports in Kazan, Russia, he won the bronze medal in the 400 metre individual medley with a time of 4:03.16.

[19][20][21][22] On 3 March, at the 2023 Pac-12 Conference Championships at King County Aquatic Center in Federal Way, United States, Kós won the bronze medal in the 400 yard individual medley with a time of 3:37.68.

[25] Day four of four overall, 4 March, he placed fourth in the 200 yard backstroke with a time of 1:39.21, which was 2.27 seconds behind gold medalist Destin Lasco of the California Golden Bears.

[32][33] At the 2023 TYR Pro Swim Series in April in Westmont, United States, Kós placed fourth in the 100 metre backstroke on day three with a personal best time of 53.83 seconds.