3 questions - 3 answers.
I have replaced Plex as a player on ATV, iPhone, iPad and Mac with Infuse, as Infuse plays Dolby Vision (although not all profiles, but that's going too far here). I only use Plex as a server. Both Plex and Infuse can basically play 4K/2160p x265.
Wifi and Ethernet: if this is a choice for you - wired (i.e. Ethernet). The bottleneck with the Wi-Fi for your files mentioned is not the AppleTV, but the Wi-Fi signal available at the location of the AppleTV (and interference from other Wi-Fi devices there).