| > I also think Thanos' durability was because Death never allowed him to die, but as soon as she was willing, a top tier punched through his chest rather easily. |
It was more complex than that, Drax had been created by a semi-abstract called Chronos and bio-engineered to be able to cut thru Thanos defences and do the deed.... Maybe at that point Thanos & Death had agreed it was time for him to die but the build up indicated it was purely because Drax was now powerful enough to kill him.
As to Thanos' chances, unprepped, against Prime - he's got none. Prime is generally way beyond anyone on the DCU earth and he's certainly beyond anyone on the MU earth, Thanos has no defence or power that could remotely slow Superboy from getting his hands round his neck.
... and considering that Superboy can travel between dimensions, even that is a slim hope. If Thanos had a lot of one-sided prep, and some serious help, he could find a way to destroy the armor and siphon Superboy's power off, but even then that's a long shot. The kid can walk through entire armies of superheroes and survive universe -destroying power.
Thanos would be hard pressed to do anything other than just running away.