SWAG77 modeled this image as a modification of Prince Louis, Duke of Nemours cavalry uniform as a Colonel.

This fanfic headcanon meta explains that Thrawn has peerage though he has been denied, and he really did not know it. So he worked his way to the top through hard work and by being highly intelligent. Unknowingly he rose to his highest position in the Chiss Defense Fleet and then was exiled, where he did the same thing with the Galactic Empire. He acquired the rank of Grand Admiral.

This image is a reflection of his ruling family forcing him to wear formal attire. The image is a wide shot.

The castle is the Schloss-Neuwenstein. And this head is by Magali Villeneuve. The rest is based on Prince Louis-Charles-Phillipe of Orleans, the second son of the French King, who became the Duke of Nemours.

SWAG77's fanfic story, eventually Thrawn becomes a Marquis by morganatic marriage to Countess Sabine Wren. That enables her to rally an "army" under the rules of the Darksaber, and him to lead that "army/fleet" and her to become a Marchioness.