Seifert competed with the Swiss team that won the silver medal behind Spain at the 2018 Austrian Team Open.[5] Then at the European Championships, the Swiss team qualified for the team final, finishing fifth.[6] He finished fourth in the all-around at the Swiss Championships.[7]
Seifert won a silver medal with the Swiss team at the 2019 Magglingen Friendly.[8] He then finished eighth in the all-around at the Swiss Championships.[9] He won a gold medal with the Aargau team at the 2019 Swiss Team Championships.[10]
2020–21
Seifert competed on the pommel horse and parallel bars at the 2021 European Championships but did not advance to any finals.[11][12] At the 2021 Swiss Championships, he won the gold medal on parallel bars, the silver medal on pommel horse, and the bronze medal on still rings.[2] He was not selected to compete on Switzerland's Olympic team.[13] He finished sixth on the parallel bars at the 2021 Koper World Challenge Cup.[14] He then finished sixth on the pommel horse at the Mersin World Challenge Cup.[15] He competed on the pommel horse and the parallel bars at the World Championships but did not advance to the finals.[16] He then comepetd at the Swiss Cup, a mixed pairs competition, alongside Lena Bickel, and they finished fourth.[17]
2022
Seifert won a silver medal on the parallel bars at the Osijek World Challenge Cup behind Turkey's Ferhat Arıcan.[18] He then won the all-around title at the Swiss Championships and the event titles on the rings and parallel bars.[2] He helped the Swiss team win the gold medal at the Magglingen Friendly.[19] He then competed at the European Championships, helping the Swiss team finish fourth.[20] Individually, Seifert finished fifth in the all-around competition,[21] and he also finished fifth in the horizontal bar final.[22] He also finished fifth in the all-around at the Joaquim Blume Memorial.[23] Due to several mistakes at the World Championships, the Swiss team only finished 20th, and Seifert missed out on the all-around final.[24] He competed at the Swiss Cup with Lena Bickel for the second consecutive year, and they placed ninth.[25]
2023
Seifert competed at the DTB Pokal Team Challenge where the Swiss team finished fifth.[26] He then competed at the European Championships where the Swiss team finished fourth.[27] He repeated his fifth-place all-around finish from the previous European Championships, this time less than half of a point away from the bronze medal.[28] He also finished eighth in the floor exercise final and seventh in the parallel bars final.[29][30] Then at the Lenzburg Friendly, he helped the Swiss team finish second to Spain, and he won the all-around silver medal behind Néstor Abad.[31] He successfully defended his Swiss national all-around title.[32] He also won the all-around gold medal at the Magglingen Friendly and helped the Swiss team win.[33]
Seifert was selected to compete at the 2023 World Championships. There, the Swiss team finished seventh during the qualification round and earned a team berth for the 2024 Olympic Games.[34] Then in the team final, the Swiss team placed fifth which was the country's best finished at the World Artistic Gymnastics Championships since winning bronze in 1954.[35] Individually, he qualified for the all-around final where he finished eighth– the best World all-around result for a Swiss male gymnat since 1950.[36] The Swiss team was presented with the Infinity Award by Fujitsu for their historic achievements.[37] He withdrew from the Swiss Cup and the Arthur Gander Memorial due to ongoing back issues.[38]