I think for Thragg's origins in the show could be like this:
At the age of 12, or whenever toddler age was logical for the Viltrumites. Thragg's potent abilities were considered similar to Argall's heirs upon several classes from teachers of Thragg. Thragg's family enlisted him by asking Argall a favor, like Thragg's family were good close friends with him at some point. In a special training camp, where he was forced into different manners of combat, for most of childhood, facing against different Viltrumites his age or physicatically, at a proper adult age, Thragg's destiny was that he would be noticed by Argall. He would try his best to be enlisted in Argall elite warrior squad, and eventually he manages to, the heirs tells Thragg he'd would be a great emperor of Viltrum someday, upon noticing the immense struggle and power he continues endure, but however he can't because not related to him, somewhat wounded Thragg's pride but still wants to try.
That's when Thadeus kills Argall, and his heirs, this upsets Thragg, somewhat pleases him at the same time, Thadeus without revealing himself to be the killer is ambiguous, and so the assassin is never discovered and leaves a mystery. Upon the council unable to find a heir, several other of the high ranking Viltrumites of Argall's elite voted Thragg to be Argall's successor, as he possessed similar capabilities and Argall admitted himself that Thragg could repserent a new generation of Viltrumites. Thragg would then command the Viltrumites to dispose of the traitor by ordering the Viltrumites to kill each other. Maybe not that last part. But that's what I think Thragg's origin could be.
