Simple things like changing sleeves or playmats should not be considered “modding the game” where the user is expected to change files inside the game dir. It’s very minimal compared to modifying game assets (actual modding).
On the settings panel a new tab dedicated to customization can be included. As a good start it would include sleeve and playmat managing. These would show visually as a dropdown of Thumbnail (auto generated) - Name. The user can browse, add, edit (rename) or remove any sleeve/playmat. This can be done in two ways. Probably the easiest way to do this would be to interact directly with the customisation folders. So no new tables would be needed (as far as I can see for now) and no older functionality would break, only thing needed are some new service classes.
Even a small UX improvement can make a huge impact and set you miles ahead of others. I think this would be a great addition in that front.