This utility mod by Sukritact allows textures that aren't included in the SDK Assets (pantry) to be extracting by taking multiple screenshots.
Just load up a game with this mod active. You will come directly to this screen
Change the texture by typing in the search bar, the image will change once you've entered a valid texture name
toggling a white BG will show a white box around the image (duh), this box will match the size of the texture
toggling the transparency thingy does that thing where you pile a bunch of transparent images on top of each other. This is done to attempt to counteract color loss from transparency
Take a screenshot with the black BG
Take a screenshot with the white BG
Take a screenshot with Counteract Transparency
Subtract the black BG from the white BG. What remains in an inverted alpha.
Uninvert the inverted alpha then apply to the conteract tranparency image
Sukritact's Texture Extractor 1.0
Extract textures from Civ 6 that aren't included in the SDK Assets (pantry)