Thanks for the tip @Sandy_Thomson 10 days later and someone apparently has restarted our TV and SYSTM won’t load again.
Because of your tip I installed Sideload Launer from the Play store and the problem is fixed! Way better than having to reinstall the apk 
Once I have started SYSTM from the sideload launcher it is furthermore working again from my native Android TV launcher. Probably until the next reboot 
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The sideload launcher also uses the appropriate blue Android Icon for Systm instead of half of a black systm logo. It displays the first wahoo screen correct and does not start the App fresh every time but resumes it and is therefor way faster at launching.
Motivated by this and the information in this thread about a screen orientation app I installed control screen rotation from the App Store.
Since a necessary permission for activating it was missing and not selectable I installed Android Platform Tools, activated USB debugging and ran “adb connect < ip address >:5555”. Next I listed all installed apps with “adb shell pm list packages” to get the package names. Now I could run “adb shell appops set < package name > SYSTEM_ALERT_WINDOW allow” with my app name this leads to “adb shell appops set ahapps.controlthescreenorientation SYSTEM_ALERT_WINDOW allow”.
Now control screen rotation had all necessary permissions and I executed it, enabled “Start Service”, “Automatic start after reboot” and set the Orientation to Landscape.
Next I was greeted by SYSTEM in a splendid Fullscreen display in Landscape! 
After starting a cycling session I sadly discovered that Bluetooth asks for the necessary permission, is displays as ON but can detect neither my KICKR, TICKR nor Cadence Sensor 
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Well at least I can finally do Yoga, Strength and MTP without having to connect my Laptop via a HDMI cable in all it’s Android glory on my Smart TV.