The issue is when you draw comparisons to Thanos and then relate to the plot point without approaching the background of either characters. That segment of first invasion was the introduction of Uxas, much before he could establish the connection to the Omega realm for power. We had the God of War Ares along with men, Atlanteans and Amazonians. Uxas was simply vulnerable to Ares who draws insane amount of power from forces in war against their foes. And he was with the defenders of Earth(not to mention Zeus and Artemis).
Steppenwolf raises the stakes when he reveals to Desaad what Darkseid had learned of the ultimate weapon “The anti-life equation” before getting to the throne. He tries to emphasize again "before ..." but gets briefly interrupted by Desaad who dismissed it for the story of defiance. But when he reveals it's the primitive planet Earth, that he’s onto, having the hidden power and how it links back to Uxas, his stonger version (Darkseid) shows up. Thanos in MCU was still evolving without power from all stones on the infinity gauntlet. He was also short of crucial stones like power, soul, mind and time for ultimate dominion and and acquiring all six stones was just seen in a single movie to actually raise his stakes for Endgame.
"But we are never told as to exactly how he betrayed Darksied or anything like that if I remember correctly." The betrayal arc is not significant here at this point since the motherboxes called out only to one of Darkseid's pawns indebted with world conquests. That pawn being Steppenwolf who had to owe his master more conquests as penance.
"So why not instead have it so that instead of Darkseid being defeated in the first invasion, Steppenwolf is the one that gets defeated and he loses the Motherboxes" The 2017 movie built on this bad idea and lost the entire potential of taking the evolving arc of a more stronger Darkseid as the actual threat, indicated in Cyborg’s vision while resurrecting Superman.
