It would be great if there was a way to do sub items of subitems. Kind of like how JIRA or ADO do User Stories then Tasks or bugs, etc. So to be able to drill down more below sub items.
It depends on how granular you like to go, I use linked boards to do that sub of sub …for as many as you like, probably neater to have sub-items separated in another board?
We have a feature info board, that I actually wish had this added level of granularity as well. We’ve currently gotten around it by heading in the opposite direction, similar to what @timyshark suggested) and essentially using Groups as Feature Set level, then Items as Features themselves, and then subitems as the Aspects of those Features. That said, it has ballooned my Group count, so it’s not a perfect setup.