I’ve proposed a solution to this which i’m looking for support for here: New Theme: Tag-Pages Navigation
i feel that the current discourse navigation breaks down when a forum has too many categories. the default view is confusing to users as there is too much going on with no obvious structure. the fixed view at this point simply has too many options. UX rule of thumb is to give a user 3-5 choices on each page.
At this point in either of those two paths, the forum only makes sense to the most committed users, as others are lost or disinterested rather than engaged and quickly able to access what they are looking for