In 2021, it was announced that he would run the first 4 races of the season with hopes of a full season in the No. 28 for his family team, RSS Racing.[1] In those first 4 races, Sieg would finish in the top-10 in all of them, including 2 top-5's at Phoenix and Talladega. On May 12, it was announced that Sieg would make his debut in the Xfinity Series in the race at Dover in the No. 90 for DGM Racing in a partnership with RSS Racing.[2] Sieg finished 34th after blowing a tire late in the race. That same weekend, he entered the East Series race at Dover for the second year in a row, finishing 13th in the RSS No. 28. In 2022 he competed part-time for RSS Racing's No.28, 38, and 39 Ford Mustangs, he would also compete for Rookie of the year honors. At the Wawa 250 at Daytona, Sieg finished 10th earning his career best finish.
On February 1, 2023, RSS Racing announced that Kyle would drive full-time in the No. 28 for the 2023 season. On April 22 at Talladega, Sieg was close to scoring his first top 5 finish after he was in the top 5 in the closing laps of the race but ultimately fell back and finished 15th.