Welcome! I've done that YVR-NRT route several times now, and I have yet to experience a better hard product. The 787 on AC is fantastic - to me the biggest difference is the air quality. Any seat you've got in J will be fine.

On the lounge - if you get the option (which is debatable), go for the transborder or domestic lounge in YVR. The international lounge isn't that great (and no showers) - but it's still better than anything from UA in my experience.

You may find Rouge a bit claustrophobic, but let us know how it goes.