"Pushover" is not correct, but yeah, even the greatest Princes of Wales would usually find themselves on the losing side of battles with their larger neighbour. As a result, it's kind of hard to justify any leader abilities that aren't going to be overly similar to similar underdogs Lautaro and Robert the Bruce (i.e. defensive war bonuses against strong neighbours).
The best candidates for leader would probably be Owain Gwynedd, or his grandson Llywelyn the Great. An earlier "King of the Britons" would be cool, but many of these early rulers are obscure and the necessary details largely lost to history. I suppose the guy with the most name recognition would be Owain Glyndŵr, also in the same vein of ultimately failed rebellion as Lautaro.
Unique unit would have to be the longbowman, especially since the English don't have it.
I can't think of a good candidate for infrastructure (a Rugby Pitch instead of the stadium, perhaps!), but the civ ability would have to be a cultural one, as Wales has an ancient literary and poetic tradition from Taliesin through Geoffrey of Monmouth all the way up to Dylan Thomas. It could be based around the
Eisteddfod and particularly boost Great Writers.
While the village's actual name, Llanfairpwllgwyngyll is certainly long enough, the "full" form (which was, alas, made up as a tourist gimmick in the 19th century) is only the
second longest single-word place name in the world
So it wasn't even worth it...