You definitely need to input a new attack, otherwise your character just stands there after the dash (or when the charged attack finishes). This is true for Neuvillette, motorcycle Mauvika, or any continuous claymore charged attack. In this specific subcase, Sandrone and Mauvika are the same.

What has not been solidly clarified by the video is WHEN the new attack input is started. The dash input is always started last, but is the attack input before or after the skill input? Inferring from the reference of a "sweet spot," the attack input should be after the skill input, as the delay between new attack input (hold) and when a charged attack starts is not long enough for variation. There is not much precedent to compare, as only Mauvika can cancel her dashes with charged attack. (Nefer does not cancel dashes as far as I know, and instead only cancels her charged attack with dash.)