Well... The DK Leveling campaign when Ashbringer is tossed to Tirion proves that the Light can be a factor against Arthas which is why I mentioned it. I added a video that quite demonstrates well, Arthas does not like the Light. And again, as a raid encounter Lich King is finally struck by Tirion, he basically neutralises Arthas, then we get resurrected to do the killing blow. Gameplay wise it's us, lore wise it's always a band of heroes but the lore figures as the main contributors. Annoying when we do all the work ahaha.
The Forsaken is probably a bad choice though originally they were part of his hand. They can wield the light and it's been confirmed by blues that it pains their existence. But since they have removed themselves from Arthas you most likely will denounce that but Scourge in general suffer from the Light - I don't need to prove that. That entire thing is everywhere. And Scourge is of Arthas making... I can't see how his creations can be hurt and damaged by the Light and he's infallible.
Here's the video:
https://www.youtube.com/watch?v=K5TUdnCIi5c - 13.42 onwards to cut the crap and sorry couldn't find a better video.
Tirion: Arthas!
Lich King: What is this?
Tirion: Your end.
Lich King: Impossible... This... is not over.
*Leaves*
Tirion knows it's his weakness too. "Your end." seems a little too... cliche but he wasn't really lying either. Arthas legged it. If it wasn't so much of a threat, like you said, he would have minced Tirion and taken him with him. So yeah the Light is not Arthas friend and proven.
It might be why he took Bolvar, to be a major project to convert him and in his own mind, overcome a weakness... Whilst demoralising people like Dranosh was for the Horde.
With Bolvar getting Tirion, he didn't specifically go himself which be either something else or he knows the weakness because as Bolvar Fordragon was kicking butt as a Paladin against the very minions he now controls.
Arthas in fact did and even mentions that he wants Tirion in his army "The others were meaningless, you perhaps..." I think is close to what Arthas said.
He went for Tirion himself, probably filled on his own arrogance and desire to have a grand army (which mocks Tirion when he delivered us amazing bunch of heroes to turn into Death Knights for his army), with being a Paladin before and most likely presumed he was going to take no damage but clearly was wrong in choosing to come, mocking Tirion then Arthas basically fled. Maybe, he never came across the Light in that wholesome degree and wasn't exposed to that weakness and why he chose to appear too rather than arrogance.
But either way, it shows to me why he left Wrathgate's. The army on his doorstep wasn't as small as Light's Chapel and there were more Paladins there with 7th Legion and well the Horde too.
All in all, Tirion seems mighty attractive to both Lich Kings without really knowing why. It doesn't seem a direct personal choice except he's a powerful foe.