My Chromecast is doing it's job fine with app only mode but the day they change it or take away that mode I've got some mini PCs ready to have libreelec thrown up on them.
It's in settings > accounts and sign in > your Gmail account > apps only mode . I have to scroll down for it on my Chromecast but if you have it on your Sony it should be in there.
If you don't have it on your Sony TV, I'd recommend installing another launcher like FLauncher which effectively does the same thing and gets rid of the garbage.