Does anyone know where I can find an official, Larian-produced changelog for the definitive edition?

Yes.

https://docs.google.com/document/d/1IcADAJsLeHHXYjCnR5MwCm9GTWRdKzRtIYcCqO_RI6U/preview

It's missing some things, though.

https://www.neoseeker.com/divinity-original-sin-ii/guides/Definitive_Edition_Differences

That's also missing a change to Guerrilla: Now also reduces cost of Sneaking by 1 AP.

Excellent!

Thanks a bunch.