In 2017, Kajaia started to wrestle for Serbia. He entered the 2017 European Wrestling Championships in Novi Sad, Serbia but defeated by the Olympic champion Artur Aleksanyan in the second round. One year later, he won his first European medal, a silver, in Kaspiysk, Russia. He reached the final at 97 kg but lost to Artur Aleksanyan again with a score of 0-7. At the 2018 Mediterranean Games in Tarragona, Spain, he lost to the eventual champion, Frenchman Mélonin Noumonvi, in the semifinals but still successfully grabbed a bronze medal. Kajaia later won his first world bronze medal in Budapest, Hungary. In the bronze-medal match, Kajaia faced former world champion and home favourite Balázs Kiss. Kajaia trailed 0-3 after the first period but managed a turnaround by winning on criteria with a four-point arm throw. In April 2021, he competed in the 97 kg event at the 2021 European Wrestling Championships held in Warsaw, Poland.