Thanks for the link! I had searched earlier for a way to affect the ordering in the New Post UI, but found threads indicating it was hard / not implemented yet.
I followed the instructions at the link you sent, and did successfully reorder the categories on the /categories page, but it doesn’t seem to affect the topmost category when posting. If the categories when posting really are sorted by number of posts, we could probably fix this by moving a bunch of Server threads into Help (which you are welcome to try, if you have time!)