This is probably a ridiculous question, but I usually stream from my laptop to my LG tv or my phone to any other tv, and I find that using my VPN keeps the casting output option from working. Like in Popcorntime, the Watch Now doesn't show my tv, only the laptop app or VLC. On my daughter's fire stick I can't even cast to it unless I turn my VPN off.
I don't have any problem streaming Plex or Emby from my laptop to the tv. Is there a way to cast without compromising my privacy?
Maybe look into an RTMP server setup. Cast pct to that, and steam from that to your TV. Otherwise I'm not sure since you want a walled garden around one thing and not another