After it took about forever to get 2 small level-shifters, I could start integrating FrSky telemetry into the project.
In addition, the control station can now display all flight parameters in metric, imperial or imperial-nautical units. To change the display units, touch the indicator in the status bar. The setting is stored in EEPROM. Also, screen brightness setting and the selected screen are now stored in the EEPROM.
The status bar now shows the APM custom mode in addition to the MAVLink basic mode and it shows RC and telemetry RSSI indicators.
FrSky integration at the moment is still in an .ino-file but I will move it into a library as soon as possible so it can be used also in other projects.