There really isn't one.

Trust me. I've worked for WBD for 4+ years specifically on the video playback component for Android devices - so my day job was literally testing streaming boxes and smart TVs.

That job made me switch from EVERY single other TV smart box, to Apple. Why, you might ask? Well, it's because ALL streaming oriented boxes suck ass. Apple is the only one who puts appropriate hardware into their products.

Lets start with Android, my field. The ONLY usable box is the Nvidia Shield, which, to date, costs upwards of £200. For hardware that was already kinda dated in 2019, when it released 7 years ago. Even Google's options lack RAM, and focus too much on the codec of the SoC instead power. This means nice playback, but awful user experience in menus and any other non-video functionality.

Your best bet, if you want to hand-roll a solution, is to buy a more expensive, high end SBC and hope for good support. You'll lose DRM, though, which means no official streaming apps (or, for the ones that do work, low resolution/fidelity playback only).

If it's Steam Link you want, get dedicated hardware for that. Even a Pi4/5 will do. Use it for Steam Link only, and keep the Apple TV for media.