So there's Deathworlders and Havenworlders, is there something in between? Those that neither had a trial nor a paradise?

Well, yes. They’re called Galactic Citizens. Most Galactics come neither from Havenworlds nor Deathworlds. You get an astonishing number of Saurians, Avians, and Lizards.

It’s just that humans tip the scales towards mammals(and Deathworlders) because we just… expand to fill whatever space we can make habitable.