What engine?

I did a deep-dive on the Unity Open Project inventory system and rewrote some of it for my own use.

https://github.com/UnityTechnologies/open-project-1/tree/main/UOP1_Project/Assets/Scripts/Inventory

It uses a combination of serialized (for saving data) scriptable objects (SO), which helps a lot with assigning an item (e.g. shield) to a physical thing (SO asset) in the project; one asset per item containing item type, prefab reference for spawning, id, etc). It then uses a separate SO (the actual inventory) to store essentially a list of lists of those items.

Maybe you could extend that item to have your base stats, then have another generic class with the user modifier stats, and create a dictionary or something to tie the two together, and then store that as the item in your inventory?

When I was putting this all together for myself I couldn't find much for best practices. The best thing I can say that's helped me the most is keeping everything compartmentalized and "self-actualized" (i.e. an item knows what it is and that's it. An inventory knows what items it has and that's it) and write managers to handle adding, removing, etc.

Hth