I've been thinking about doing an episode on request.js. It's one of those things that I would like to see it as a default in new Rails apps, but also seeing the fetch/promise stuff is good. But, I agree, it is cleaner by far.
Ah... I think I see the problem. In your code, you're closing the div before the form select for the secondary target. So, the secondary target is NOT part of the stimulus controller.