Roman Empire at their best cant even conquer all of the Germanic tribes, lose to the Goths, how can they beat the Huns who were way stronger and have way better tactics and weaponry than the Germanic tribes? Dont write things that you dont understand. You are clueless about the Huns.
You focused too much on the 451 war. The theory is not West Han > Huns > Romans but more West Han > Huns > Germanic tribes > Romans. The idea is that the Western Roman Empire was destroyed by the Germanic tribes and it was the appearance of the Huns in the late 300s and early 400s in Eastern Europe that drove the Germanic tribes to seek refuge in Roman territory leading to wars between Germanic tribes and the weakened Roman empire. Of course, separately there is a debate on are Huns = SiuongNu or not.