Step 1- make shields crafted from swords; i.e. "Final Flourish (DualWield)" = a shield. Just a shield.
Step 2- make up stats, or copy stats from existing shields, then give it to those sword-shields
Step 3- Optionally, make the knight do the sword's charge as a shield bash
Why not have the sword-shields resist the same type of damage/status they deal? Would make it a bit easier to work with.
We can make a sort of formula for it too! How about the weapon's shield regen rate is dependent on the weapon's attack speed (troika would be slow, cutter would be fast). To balance this out, the damage that these weapon shields resist would be a fraction of their damage dealt, the numerator being 1 and the denominator being the amount of clicks in a combo. Troika would have 1/2 its damage in defense and a slow regen rate. A flourish would have 1/3 its damage in defense and a fast regen rate. Cutter would have 1/5 its damage in defense and a super fast regen rate. Brandish would have 1/3 its damage(s) in defense and a moderate regen rate.
This also opens up the potential for a "Shield Regen Rate" UV that is applicable to these shield types, especially if the shield is crafted from a weapon first.
Lastly, I would say the charge attack as a shield bash is a great idea, but would recommend they do reduced damage. I'd think that the defense value being the damage value (which was the damage value in the first place) would be a fair way to do it.