Plex Multiroom setup using nvidia shield and firetv4k as clients

Correct. Recordings will in fact be stored in whichever Plex Library you have set to contain either TV Shows (one kind of Plex Library) or Movies (another kind of Plex Library).

Possibly with this:

Though I don’t get the sense it’s easy to setup/configure

Read through Support Articles regarding Live TV and DVR. Sign up for a monthly Plex Pass instead of shelling out for Lifetime Pass before you know if it will all work the way you need it to.