How do I subtly hint her that I may leave her and move on?

It sounds like the path you chose to avoid wrecking your friendship might be causing it even more harm than if you had committed. By saying you were just going to "let feelings develop," neither of you really knows exactly where you stand, which causes tension and anxiety. You should just decide, right now, whether or not you really want to be with her. If you do, tell her so. If you don't, tell her you like her better as a friend.