As someone who knows a lot of theory, this is an excellent explanation! One piece of advice I would like to add is to keep your melodic ideas simple. Look at some of Nobuo Uematsu's work on Final Fantasy VI; Terra's theme is based around a simple two-note walk up at the end of each bar of music. He takes this super-simple idea and messes around with it in a number of ways, then takes one of the variations and does variations on that. Or think of Beethoven's 5th, where a good chunk of the piece is based around THE EXACT SAME FOUR-BEAT PATTERN REPEATED OVER AND OVER. It's amazing, and a good way to develop a catchy harmony that will stick with the player long after they shut off the game. It also helps as a stepping-stone toward learning how to create more complex melodic ideas; once you've mastered the basic and the simple, the advanced stuff follows naturally.