From my point of view the deep forum structure will not lead to more contributions than a flat structure. With a flat structure people can post topics even if their post doesn't fit in 100%. With a strong hierarchical structure there is no "air to breath" for thoughts and ideas. From my point of view - which is over 10 years of professional website building - a flat structure drives the people to ask and answer.
I suggest to delete at least the deepest level and make sure that we have many terms in the first level.
The hierarchy is now streamlined. It is far better to navigate and to understand.
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