How do Sword Dance stacks affect Thornblade?

The 180% is a ceiling, not what every swing does. Stacks are spent when you attack, so the buff sits on a meter you build and then burn rather than a passive that runs all the time. That makes Thornblade a character you hold rather than mash. Swinging the moment a stack lands spends it at the bottom of the range. Letting the meter fill first puts more of that increase on the hits you actually care about.

The obvious place to dump a loaded meter is a broken enemy. Depleting a target's break gauge shatters its shield, interrupts whatever it was casting, staggers it, and raises the damage it takes by up to 200%. That is a window where the target cannot fight back and every hit lands harder, which is exactly what a stored basic-attack buff is for. Spending stacks into a healthy, unbroken enemy is not wrong. It is just worth less than the same stacks spent a moment later.

One thing to expect: your Thornblade and someone else's will not hit for the same number. Personality determines an Aniimo's stat growth and distribution, and each letter of the personality type grants a small stat boost, so individuals of the same species end up with different bases. The percentage is the same for everyone. What it multiplies is not.

Not settled: How many stacks are needed to reach the full 180%, and what builds them in the first place, is not pinned down in the closed beta build.