I do not think one-handed fighting style should be on par with sword and board in terms of AC.
However i do not see a unique advantage the style has over other options and i think this could be addressed in a different way.
Two-handed style offer high spike damage, due to weapon dice and str scaling.
Dual-wield also offers high damage, this time due to more attacks, which works favourably with additive damage like sneakattack, divine might, bard song etc.
Sword and board sacrifices damage in favour of the best defense, be it trough shield parry and/or being able to further increase shield AC with magic vestment/enchanting.
One-handed style in comparison offers less AC then sword and board even with all three feats, while putting out the same damage.
IMHO it would make a lot of sense to give one-handed style an advantage in AB. Not being encumbered by a heavy shield,
twohanded weapon or another weapon in the offhand, should give an advantage in accuracy.
Each of the feats could for example give a +1AC and +1AB when the offhand is empty, or not having an offhand equipped could give a +2 to AB in general.
This would greatly benefit the classes that tend to opt for such a style of combat, generally low/medium BAB classes (bards, rogues, the occasional wizard),
while not making it straight up a superior choice, just a trade of between damage/AC/AB.