I dont understand why you want to keep a TV offline for privacy/ad reasons, but also say you’re using a Chromecast or Roku media devices anyway? Thats like wanting to keep your windows locked but leaving your front door wide open.
Anyway, LG and Sony OLED TVs offer the best image quality and will work just fine without an internet connection.
I have my LG TV offline and use an AppleTV for everything. I never see the LG OS or Home Screen or any ads or logos once it’s configured to boot straight into external HDMI device.
I’ve been in similar situations while renting. I ran ethernet cables along skirting boards and around doorframes and hid them inside adhesive cable raceways.