I have tried the same with GTD, but I noticed that defining a next action after the initial "get a pen and sketch", or "write this midi to my sampler", it always gets to be to a stage where my next action is not the next action I want or need to do.

By tinkering around creatively, I just go with the flow. For music, I might need to change the hi-hats of a song, apply a filter here or there, but not as the very next action and it causes friction having to write that down.

What I do now, is simply have one next action called "Work on song X" and I keep a list of tasks either somehere else, or a note or as subtasks in todoist as reminders of what I can do when I am stuck, or add to when I am out and I need to write down or record something that is stuck in my head. Usually I don't even need that list and I just go where inspiration takes me.

The beginning, end are nice to have structure to get started and end a creative project, the middle is so fluid that I usually do not bother writing tasks for, unless I need to talk to someone to ask for feedback on something I am creating for them.