I wouldn't do anything to arouse suspicion if you plan on keeping some sort of relationship with family after you fade. No asking questions or addressing doctrinal issues. Make it seem as if you're just going through a hard time personally, in general, with your life - that seems plausible for JWs without getting their "apostate" radar tingling.
I spent most of my teen years double-lifing and completely zoning out at meetings so i know it can be done. Think about plans for the future- do your bills- plan a vacation- just occupy your mind while you're at meeting. Think of it as free thinking time for yourself. I also had my own car so I often came in right when the meeting started and left as soon as it finished. That avoided unnecessary conversations and questions/comments about my lack of enthusiasm. Literally, thousands (if not more) of young adults have done this before you- so many in my hall turned 18, left their house and were never seen again.