Recently got a Google Smart TV for the first time, instead of Roku, and I hate it so much. Roku interestingly I think folded in ads in the most non-obtrusive way (except for the full screen ads which I think were quickly abandoned). But Google Smart TV is a completely intentional bid for sticky integration that fosters Google dependence (google login, google telemetry tracking what you watch inside of other apps, other streamers are google apps), which is not how I want to experience my streaming. It's also slow and sometimes glitchy. I had never had a TV capable of crashing before.

Roku at least felt non-evil or non-evil adjacent in its notional neutrality.

Google and Apple seem like the best competition and they do have streaming services, although Google's is just their bad YouTube tv thing and very ignorable. I'm not sure Amazon is even in the running now.

The Nvidia shield used to be a decent streaming box?

Look into the TiVo Stream 4K. It’s an Android box but has been very reliable for me. Tivo does force some quirks so I used ADB to disable core services and the default launcher has ads so I switched to Projectivy launcher.

You can go to Walmart and buy a streaming box that is a Raspberry pi-sized board with custom Android installed and the package claims it has 700+ channels. But it just is an overlay for pirate streaming sites.

An android tv you can buy them for 20$, and put any apps

Not really. Apple TV seems to be the closest ive found to not being riddled with ads though. the home screen doesn't have ads at all, the closest which exists is the "top shelf" feature when you hover over the Apple TV app, and that can be turned off in settings. But it has some other issues