I wonder if the BJT connected to the OS signal would be neccessary, if an operational amplifier in voltage following mode was connected instead.
Since the output is an analogue voltage signal, I got the impression, that the BJT might have a role in linearizing the signal.
My (simplified) circuit would connect the TCD to an ADC as in 3. In the real circuit there would be additional analogue amplifier stages as per the ADC datasheet, but notably it is lacking the BJT.
As suggested in the comments, I will keep the BJT. Since the datasheet does not specifiy any particular model, I'd like to extend the question to which BJT model should be used. My choice wold be 2N3906.