Hmm...
I like Thanos with the prep in round 1.
In round 2 I'm kind of torn. Thanos isn't stupid. What's stopping him from simply regrouping if SBP rushes him and gets the upperhand? He can teleport interstellar distances after all. Thanos fights smart and plays for keeps. I don't see him giving SBP the opportunity to finish him off straight up even in a random encounter. He'd find a way to cheat. It's what he does. Like on the fly booby traps, leading SBP to a stellar event like a wormhole or something to buy himself time, playing mind games with SBP. He'd be calculating and ruthless. SBP never showed me that he could be cold and calculating. He was just crazy, powerful and immature. He'd be the first to make a mistake in my opinion.