Early career Štummer made his professional debut on July 23, 2022, against Bartosz Rewera. Štummer won the fight via a Unanimous Decision. His next fight came on February 4, 2023, against Gergely Csibi. Štummer won the fight via a third-round submission.
Konfrontacja Sztuk Walki Štummer made his debut under
Konfrontacja Sztuk Walki (KSW) on April 22, 2023, against Jonatan Kujawa. Štummer won the fight via a Unanimous Decision. His next fight came on October 14, 2023, against Dawid Kareta. Štummer lost the fight via a first-round knockout. His next fight came on February 17, 2024, against Adrian Wieliczko. Štummer won the fight via a second-round submission. This fight earned him his first Submission of the Night bonus. His next fight came on October 19, 2024, against Wojciech Kazieczko. Štummer lost the fight via a third-round submission. His next fight came on February 21, 2025, against Nacim Belhouachi. Štummer won the fight via a first-round knockout. This fight earned him his first career Knockout of the Night bonus. His next fight came on August 9, 2025, against Eduard Kexel. Štummer won the fight via a Unanimous Decision. His next fight came on February 21, 2026, against Michał Domin. Before the bout began, he was deducted one point due to missing weight. Štummer won the fight via a Unanimous Decision. ==Championships and accomplishments==