Sure, 2 AP would probably work as well. I was not sure about the number myself and in the end looked more at the benefit with a thunder-forged weapon.
Just give it any drawback for Str builds, otherwise it would be a clear best choice with thunder-forged weapons. Which is the only reason I can see why this has not been added in the first place. I can somehow understand Scimitars, since bards are not proficient, but Longswords are an iconic bard weapon as well as a weapon we would associate with the Swashbuckler style.
You can call it Swashbuckler of the old school or something like that and afterwards you will no longer have to listen to our complaints about the weapon choices![]()