I don’t know if you’ll like this idea or not, but I created a personal project called “Follow-up” that I add all my follow-up tasks to. I wrote it up awhile ago with more details here:
The reason I’m not sold on the idea of follow-up tasks being subtasks is the implication that subtasks are subitems that are required for the parent tasks to be considered complete. Admittedly this is my viewpoint and people are free to use sub-tasks however they like, but I still feel like that’s an argument for why Asana wouldn’t automatically make them a subtask and it would actually cause issues for the way I use them. Is it difficult to make them subtasks after the fact? I might be able to offer advice for that as well.