Instead of Persist you are probably looking for global items or a global layer. This way the objects would continue between layouts. The global layer is more for a HUD or UI if the inventory acts like this. Making the inventory slots themselves 'global' by ticking the box in the properties means they will travel between layouts. Don't use both the layer and global object, pick one style.
In general though you should be using Arrays. There is a tutorial here although it might be a bit scary to look at if you are new to them. construct.net/en/tutorials/building-rpg-style-inventory-2499/programming-inventory-setup-4
Think of the array like a list or table of items and the rows are slots for storing them, columns would be data related to the item like name or amount. It gives you less of a physical style inventory and more of a storage that you can manipulate with events, you have more control with that.