Ha, S2 was at primary school with a kid called Tristan.
Whenever he was mentioned among our family and wider friends I would add "...nothing like what you're imagining".
He appeared to be a decent enough child, and was actually relatively good at maths, but was definitely one of the Tough Kids, had an uncle who was in prison, and played in the local football team that another parent described to me in less than glowing terms...
So it appears that the name has transcended its origins 
I would put Lysander and Sebastian in the same Definitely Not Tough box. Edward more normal but probably friends with Sebastian (they call each other Ned and Seb).
Marius doesn't have such connotations but certainly where I grew up is homophonic with marry us so I'd suggest not.
Thorfinn is sufficiently unusual to mean nothing more than Bit Foreign on first encounter. Possibly in the same class as Otto and Luca, two boys I know who are also Definitely Not Tough.
I quite like Magnus. I don't think it has to mean tall, the greatness can be metaphorical. Mind you I'm old enough that it means dramatic music and lighting.
I'd go for Magnus or Thorfinn.