Thank you!
For the inventory I used 8 variables to track if the player got the different items and toggled the sprite actors according to that.
In the inventory scene, the player changes to an arrow and cannot move due to invisible walls.
I track the selected item with another variable.
Then the player can move to another item by pressing the according buttons.
I had to write a ton of if-else to do this manually and it's just a mess in the editor :D