AppleTV is the media box to use if you hate ads and value your privacy. Its Home Screen is just a grid of apps. No ads. None. It’s also way faster (CPU speed) compared to the competition.
Replacing a smart TV OS with a device made by Google or Amazon defeats the purpose. You’re still going to get ads plastered all over your Home Screen. And they still lag and stutter unless you pay a premium price for an 5 year old Nvidia shield. Either use an AppleTV or build your own HTPC.
Seriously, buy an AppleTV. It works standalone without the need for other apple products. Has a fast processor, ability to disable telemetry, good track record of software update support etc. and NO.ADS.IN.THE.OPERATING.SYSTEM.
Using an Android box will result in the same issues you’re trying to get away from with SmartTVs.
If you’re technically capable enough you can build your own HTPC, but due to DRM you’re going to run into issues streaming 4K content from streaming services. And PC and Linux HDR and Dolby Vision support is a rabbit hole.