[3][4] He contested the final two rounds of the 2022 Italian F4 Championship, also with Monlau Motorsport, competing under a Belgian license.

[5] On 5 January 2023, it was announced that Koolen would be driving for VAR-Pinnacle Motorsport in the 2023 Formula Regional Middle East Championship.

[6] Following the 2022 season, Koolen partook in several Formula Regional European Championship tests, driving for Van Amersfoort Racing.

[7] He would ultimately sign with the Dutch-based team to participate in the 2023 championship alongside Joshua Dufek and Kas Haverkort.

[16] Alongside his Indy NXT season, Koolen made his endurance racing debut and competed in the 2024 European Le Mans Series with the Duqueine Team.