I've been using a Raspberry Pi 400 with LibreELEC installed. Mostly watch 4K HDR Blu-ray Remuxes that I have on another machine with a Samba server. Works really well for me.
Another good option would be to have Jellyfin on a media server and cast to the TV or use the TV directly if it has a Jellyfin app (I know there are official apps for Roku and WebOS (LG)). Jellyfin is similar to Plex but open-source and fully local (no need for an external account).
Of course, this is only works for local media. For streaming, just use a Chromecast.