As for why Thanos does it this way rather than exploiting his powers to the full at each stage, I think that's mostly because this particular narrative is more interesting. Remember Thanos here is the hero of the narrative. He thinks those who oppose him are simply wrong, not evil. He probably sees groups like The Avengers as performing a useful role in maintaining the balance of the universe.
For all his talk of "mercy" and "balance" - Thanos is a sadist. People kept asking - why doesn't Thanos just double the resources? It's because he's not that type of guy. The Russos hit you over the head in Infinity War that Thanos enjoys inflicting pain and suffering.
Ebony Maw - "let him have his fun" as Thanos systematically dismantles the hulk
Choking out Loki slowly before breaking his neck (seriously, that was pretty intense for a move that kids are going to see).
The flashback of executing half of Gamora's village - while the other half is looking right at them!
Forcing Quill to choose to kill Gamora, even though he knows the whole time it's not going to happen
How long he goes on torturing Nebula, even after Gamora confesses the location of the soul stone he's not going to let her off.
Thanos' logic about how to save the universe is flawed because he's fundamentally a bad guy - someone who feels slighted and needs to prove he's right. Thing is, he's not going to end it quickly unless he has too (like needing to GTFO after the snap), he prefers to twist the knife.