If I have an Ancient Pokemon in the Active Spot that attacked last turn and I use Thorton to switch it for a Koraidon with "Unrelenting Onslaught" attack, would I get the bonus damage for "Unrelenting Onslaught"?
No, all effects carry over when using Thorton, so the game sees Koraidon as the same Pokemon for all intents and purposes, including having attacked last turn.
Source: TPCi Rules Team (2024-12-19)

Relevant cards

Note: All applicable cards may not be listed below.
SV8_116
SV8_116
SWSH11_167
SWSH11_167