The subject of working from imagination after primarily working from reference is something I've spent a lot of time on, because the two modes of working require different cognitive approaches.

Reference work is observation and translation. You're parsing visual information and re-expressing it in your medium. The challenge is accuracy and interpretation.

Imagination work is synthesis and invention. You're drawing on stored visual information and constructing something that doesn't exist in front of you. The challenge is access to that stored information and the ability to check your constructions for plausibility.

The pathway from one to the other: draw from reference with deliberate attention. Don't just look and copy — ask why the thing looks the way it does. Then draw from imagination immediately after, while the observation is fresh. The imagination drawing will incorporate what you just analyzed. Over time the analysis becomes automatic and the imagination library expands.