I personally just adjusted the default KDE desktop to look decent on my TV, since I set it up before the current release of bigscreen did. Bigscreen just seems like less tweaking overall, which seems to fit what you’re looking for. I’ve tried other options like Kodi, Android TV and ultimately been left disappointed. Using KDE connect (or a controller with steam input) and a browser is definitely a bit janky, but it does give you the freedom to use whatever media platform you like without worrying about whether there will be supported apps or plugins for your smart TV/Box. You aren’t going to find a TV Box/Smart TV that isn’t spying on you, at least out of the box. I fought this for a long time trying to find something that worked better for me.
YouTube’s default UI isn’t too bad, in my experience, admittedly my tv is on the small side. I use slightly increased scaling to make things bigger. I use jellyfin and it’s default web interface works well on a tv, I haven’t tried the desktop app but it’s there. I used to use Kodi and while it did present a much more TV centered UI, getting each app working was a matter of managing different third party plugins that would break on me frequently. I used to use Freetube, which both blocks ads and includes sponsorblock. It lets you use YT mostly without an account, including subscriptions, playlists, watch history, etc. It takes some fiddling to import your subscriptions if you do use a YT account.
Using the mouse and keyboard on my phine via KDE connect is my default and works well for the less TV centric UI.