One of the bad reputations I inadvertently acquired at my last job, I'm told, was that I'm a technology zealot. The charge is that I care more about how good the tech could be, than about delivering actual value to players.
I understand why people might think that about me, because, I really love thinking about awesome tech, in particular workflow tooling. I might be fanatical about instant feedback and safe iteration.
In order to keep myself in check, I've developed a couple of personal rules.
Rule 1. Never make a tool for a user who does not exist. This means I need to figure out who, exactly, will be using the tool, and make it work for that real person. This keeps me from getting lost in tool usability land. If I'm the user, and I know how to edit a json file, great!
Rule 2. Prefer viewers to editors. Editing a text file is actually a pretty rich experience these days. If you can edit a text file, press save, and see instant feedback, that's pretty good. You don't necessarily need or want to drag boxes across the screen. This rule alone has saved me hundreds, maybe thousands of tool programming hours while working on Legacy.
In-place runtime iteration is the gold standard. You should be able to modify source files to edit any number or string value you care about, and instantly see it updated in the game. If you can do that, you don't need a dedicated editor, in-game or otherwise. JSON is a pretty good data format. In particular, it's a good data format when you're editing it inside an IDE with live syntax feedback and domain validation.