Hi jonboylib
I go through the post carefully, I am sorry to say there could be no built in feature to create subitems in a List in Microsoft List or SharePoint list, however, create a group view in the list could be a good workaround, but we need to record data like screenshot below:
Then we may create a new view in list with configuration like below(More info on Create, change, or delete a view of a list or library (microsoft.com)):
After switch this view, we can get something like subitems
Additionally, If you can save information in different lists, you may try using Power Apps to create Cascading lists: Working with Cascading Lists in SharePoint and Power Apps | Microsoft Docs
Best Regards,
Clark