@georg_laage It seems that with the latest version of TensorFlow 2.5.0 you do not need the below lines of code:
converter.experimental_new_converter = True
converter.target_spec.supported_ops = [tf.lite.OpsSet.TFLITE_BUILTINS, tf.lite.OpsSet.SELECT_TF_OPS]

You can make the conversion like this 8 code snippet.
Can you confirm if you have time?