Thank you so much for these super exact dimensions, you saved me hours of headaches <3
I went for the 10.65 : 0.95 ratio, and GIMP accepted them to the fraction, yay!

As for the darker middle: create one layer, spanning half the image size. Fade it from pitch black (towards the middle) to fully transparent (towards the side edge). Then simply duplicate that layer and flip it horizontally.
Lastly merge these layers together, and set the resulting layer opacity to somewhere around 65-100 % opacity, according to taste.
Turned progressive rendering off, and went for 98% quality, which comes to 363 kb file size @ 1920x172 .
Considering the file size, I am really happy with the result.