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.