All I can really say is that a number of the developers themselves do everything through the UI too. I’m personally satisfied with the UI for most things and it’s getting better over time.
As for the clicking around, see OH 3 Tips and Tricks, in particular the “Buying in Bulk” sections. Once you create the exemplar you have options to do “copy/paste/edit” type stuff to create more. The options are a little bit more limited for Items but the REST API and messing with the JSONDB directly are both options.
I also sometimes use “Create Items from Text” to create a bunch of similar Items using .items files syntax. I don’t know if you can edit an Item that way though.
The UI supports both creating Items and rules. Obviously you can do it in .items files if you want and if you prefer but nothing says you have to. There are lots of ways to create Items in the UI beyond the “Create Equipment from Thing” option in the Model View.
Also, you probably don’t want all your Items in the model anyway. See A Deep Dive into the Semantic Model