Taxonomy management

WordPress has had custom taxonomies for years, but there is no way to manage them. While built-in tags and categories can be converted into each other, they cannot be converted into custom taxonomies or vice-versa.

The result is functional uselessness of custom taxonomies for any site which has already made extensive use of tags and categories.

Further descriptions of this problem:

• https://justintadlock.com/archives/2010/06/10/a-refresher-on-custom-taxonomies#comment-201873
• https://sillybean.net/code/wordpress/convert-post-types/comment-page-1/#comment-33466
• https://stackoverflow.com/questions/4167706/wordpress-custom-taxonomies-conversion-and-filtering
• https://www.ads-software.com/support/topic/splitting-out-old-tags-into-new-custom-taxonomies

Currently open support request looking for a solution to this problem:
https://www.ads-software.com/support/topic/convert-tags-to-taxonomy-tags?replies=13
After searching for weeks, it is clear that there is no solution at the moment. An opportunity for a plugin developer looking for a new project? Or a feature to add to any of the existing tag/category/taxonomy plugins?

It is also seems like a glaring need within WordPress. I don’t understand why custom taxonomies are in the core, but the ability to make use of them is not.