Roku is really locked down. There’s ads on one side but nothing else.

Otherwise Projectivy is an Android TV launcher that can be configured to be really stripped down. It takes a bit of time but if you do it right it’d show less options than even Roku (it’d show only the apps you select, no launcher settings etc).

As a box I’ve heard good things about Onn (Walmart) if your in the US. If not, Homatics is great but pricey. Kick Pi KP1 is more affordable (but still way more expensive than Onn).