I have a MI Box S with the android tv 9, but I couldn't find a way to "clear all" running application on this device. All I can do is to go to info section of each app and click on "Force Stop".
I also installed "button mapper" app, but there is not a "clear all" action in its options.
Is there any way to clear all running apps with a single action on this device and android tv 10?
You really don't need to. The Android OS manages RAM use well now and will automatically kill running apps as needed. The only thing I ever do is occasionally clear cache @ device storage settings.
I'm not sure about the Mi Box, but on my Shield double pressing the "home" button brings up recent apps where you can close apps from (although only one at a time). Since the Mi is also running Android it should be the same I would think. It's better that the way you are currently doing it.
I have Fast Task Killer installed on mine... Seems to do the trick for me https://play.google.com/store/apps/details?id=jp.snowlife01.android.appkiller2
If this a RAM issue that you trying to deal with then iI suggest installing 3C Task Manager and Android Assitant. They can be set to automatically kill apps and free up RAM. Apps will still be in a recently opened state but are not preforming in background. It has solved my RAM issues. Also SD maid I use to auto clear cache. This trio is the best that iI have come up with. My Box was been running flawless with this setup.