When the output from my BLE app arrives on the arduino its just gibberish

Extensions
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2

Try send
ServoTumbPosition join \n

Another idea:
https://community.appinventor.mit.edu/t/ble-esp32-bluetooth-send-receive-arduino-ide-multitouch/1980/17

Use Slider extension:
https://community.appinventor.mit.edu/t/free-open-source-slidertools-bit-of-useful-feature-for-sliders/17459