I'd second the use of Kodi since Plex requires internet.
But that said if your media is already in iTunes you might get a mac mini and use it to share your media, then you can either use the ATV1s or pick up a new ATV3 or 4. They can then use the "computer" option to stream data.
At home I use the mini to serve my media throughout the house and use the iTunes for some media and Plex for my older media. Plex is able to run on my FireTV and ATV4 without playing the sideloading game.
However in our RV I fall back and use Kodi since Plex requires internet (which is a bummer). The mac mini serves the media right to my main TV, and the rear TV has an oder ATV with some media loaded on it, and the ability to access the rest via my in RV wifi, which just allows me to stream, but that is only used for loner stays.