It is better to get official Android TV device like Mi Box S to have official support of Google Assistant, Chromecast and YouTube 4K.
Some generic boxes are said to support Google Assistant as well but the experience may not be as good.
Eg: Beelink GT1 Mini which comes with a USB Wireless Remote claims support of voice assistant. But the implementation may have problems.
Ref:
http://www.bee-link.com/forum.php?mod=viewthread&tid=45702
No point buying a new tv box to get the microphone to work.
Just want to find out what additional apps needed to get it to work.