So you are saying any cellphone will do 4k with audio passthrough but you haven't tried by yourself.
The majority of phones doesn't even do video out through USB-C let alone pass 4k hdr + atmos audio, maybe if you have a galaxy s20'something (one of the few phones that has usb-c dp alt mode) it might work but most probably it won't, the other guy is a moron.
What you want will be problematic with a custom x86 android machine, but you could also use the native tv apps just for that or a different media stick like a nvidia shield, google or fire tv for example. I think the use case you're specifying isn't very important for people running custom htpc's because they rely more on their own unrestricted sources if you catch my drift