1. How do I maintain interest/curiosity? I do not. I lose momentum and interest. I get depressed. So I pull out a blank sheet of paper and draw a progress meter. I commit to 20 hours of practice - dedicated either to a new skill or to generally overcoming my stagnation. I have done this dozens of times, and it has never failed, I always regain my passion.

2. How do I avoid going all in? Going all in is my default. I treat the 20 hour mark not only as a goal, but as a stopping point. I must make the conscious decision to draw another meter in order to continue, after considering how my time is being spent - otherwise I can become obsessed and lose balance in my life.

Note that your phrasing is self-defeating. You are telling yourself a story which appears maladaptive. Just listen to yourself: 4 months of valuable experience was an "evil" that you had to "knock out" to the "detriment" of "actual" practice.

You identified a weakness: You knew that you needed to develop your ear, and make sense of the territory. You might have collected a hundred brief sound clips and challenged yourself to identify the sub-genre. You might have randomly selected a genre and challenged yourself to produce a very brief (even minimal) piece that expresses its essence. You might have taken a short composition and, for any given genre, challenged yourself to reinterpret your own song in that style.

Here's an idea. Take 5 different genres, and challenge yourself to making a short piece that gradually and naturally transitions between them. Optionally ignore all other constraints, like global structure, general appeal, consistent aesthetics, etc.

You did 4 months of practice which was in all likelihood less than ideal, somewhat inefficient. So what? That's a positive, not a negative. Next time, try to incorporate "doing the thing" into your regimen and you won't feel like you've lost momentum.