Question How to allow registered users to create a sub-category within a category?

Please see the hierarchy:

Main Forum> Self-Help> Boundary>Don't read this book

"Self-Help" is a book category.
"Boundary" is a name of a book.
"Don't read this book" is a review of the book of Boundary.

Now the registered users can only create a "topic" within each category and the admin can create a sub-category from back-end.

However, I would like to allow the registered users to create any sub-categories they want within Self-Help at front-end. How do I configure?

I'm sorry, but it's only possible to create categories in the backend.

Thanks for reply. However, are there any paid features can solve this issue?

By the way, once I moved a category to top-level. It becomes a "section" and it disappeared from the "category list" , where can I find it back?

Thanks for reply. However, are there any paid features can solve this issue?

That's not a easy issue, it would need many time to develop this extra feature. You can deposit your request here /forum/custom-work-offering-to-pay/ , but I'm not sure whether you get a feedback.

By the way, once I moved a category to top-level. It becomes a "section" and it disappeared from the "category list" , where can I find it back?

It seems this bug: /forum/k5-1-support-archive/156055-can-t-see-categories-anymore-in-backend

so you have to creat a new topic ? DigitalOcean SiteGround iPage

so you have to creat a new topic ?

I am hoping to set up a category (a name of a book), then each user can create a "topic" within that category , which is their review of that same book.

However, I also hope each user is free to create any category, which means they can open a category for "any book", and they can wrap all their reviews (topics) in one category (name of the book).

Do you have any way which may have the similar effect?