As an adult learner, what should I work on first for using roots without guessing the meaning of every derived form?

For using roots without guessing the meaning of every derived form, choose frequent roots from verses you already study and compare a few related forms. The aim is recognition in context, not memorising a large dictionary of roots. For this focus, keep the target narrow and stop the drill once accuracy starts to fall. Look for recognising related words in new ayat and using context to refine the meaning.