No I don’t think you are, this OH3 move has been kicking my ass too.
I’ve all my stuff still in .items files, as they all work there and I’m used to editing them.
I think you either have to keep editing them there and add any additional info you need to work with OH3 like Semantic Class and the like, or you start over and make new items completely through the GUI.
In your first screenshot you have an Add Metadata option. There’s another thread open in this forum which asks the same question, but at the moment you cannot see your custom metadata within the UI on the pages you’ve shown.
What happens when you click Add Metadata and then choose whichever option has the word Custom in it?