Shields honestly need a defensive advantage, not an offensive one. After all, defense is what they’re for. One or two more Stamina and a wider block arc do not compensate for the lack of mobility with shields. Bosses and Elites can beat down your defenses with one hit no matter how much Stamina you have, and Bosses also have attacks to damage you through your block. Disablers also need to be dodged, they can’t be blocked. Even when you know about it and can manipulate it, hyperdensity will occasionally happen and that will chew through your defenses just as easily. All these cause the very low mobility of shield weapons to be very punishing. My suggestion would be to address the first two by giving shields special resistances, and to try fixing the hyperdensity further.
There certainly is offensive trouble with most shield weapons too. That’s more a matter of huge difficulties against armor and hard-to-use strike patterns (bash as second heavy is very hard to use consistently and effectively) than any general killing power. Address those, and the offensive power of shield weapons is close enough to their unshielded counterparts to be usable.