You can sideload virtually any android app. Some may require an actual or virtual mouse to deal with the default touch screen navigation. There are a variety of tips and hacks found in this forum to allow these generic android apps to be seen on leanback launcher home screen.

Install the Android TV version of the Aptoide App store:

https://www.aptoide.com/

Also install the Sideload/Launcher from the inbuilt ATV Playstore to access those touchscreen Apps in a folder.

The Rotation Locker Apps come in handy too. Plus a Wireless Air mouse of course.