is there a way to add sub items to sub items?
I believe there are some third party apps that can accomplish this, but natively within Monday there is not technically a layer below subitems.
What our team does is uses checklists in the updates of subitems to act as a layer below sub-items. If you want to learn more about checklists here is an article from Monday https://support.monday.com/hc/en-us/articles/115005342005-The-Checklist - this option is great if you are looking for something that does not cost extra and is natively within Monday, the third party apps will likely have a cost associated with them, but might offer some additional functionality.