I have seen some other people mention Airscreen for Android although I've not tried it myself I think you can download it as an app, the free version has various limitations but would probably give you the chance to try before you buy it.
If you ended up having to buy a box and don't want to spend Apple TV money, I've just bought a Sunvell T95Q which has
AirPin (Pro) installed on it ... not sure if that's available as an App or not.
I've only used it portrait for photos so far but if you had any interest can test it this evening and will try with videos and sound.