You would need at least 6 Athletics and 5 Shield to get the most out of it. They're REALLY small. If you go for a more typical approach in teamfights, you'll always be hit from the sides from weapons and ranged. With low shield hitpoints, you have to pick your fights very carefully and not take unnecessary hits that don't give you better positioning. Loomed bucklers and more shield skill will definitely give you some noticeable leeway.
Although, it's not a matter of what shield skill you need for a buckler to not break, because it likely WILL break and it's what you need to do in battle to take advantage of its unique perks while you still have it. Avoid bonus to shield weapons at all cost (high athletics will help). Try to outlast the enemy team, when there's less people overall in the round, that's when it'll shine, during 1v1s.
I think it's a fine line for the Steel Buckler and may or may not be worth its upkeep. It can take more hits, which makes up for the Round Steel Buckler's weaknesses, and it doesn't weigh significantly more, but it's slower, and costs more than twice as much.
