I have a client with a lumberyard who needs Lumber to be a category with subcategories. We would like to use category-lumber.php for both the parent category and the subcategories. I found this post from 2010 that addresses the same question, but was not able to make it work: Make all subcategories use the template of its category parent? It could be that I'm using it wrong, or that the functions are totally out of date.
You can see the goal on https://goosebaylumber.net/category/lumber. Currently the subcategories are picking up the default archive.php
Update: now that the issue is fixed, the link given is no longer showing the default layout. But, it's the same layout as the blog, which is using archive.php.