When you say agitate, you mean the drum rotates when no clothes are in it.
Can you manually move the drum by hand when the door is open, and does it feel free (not stiff)?
It sounds most likely the drum belt is starting to fail, this may just need to be adjusted to tighten or need replacing all together.
Can you advise me of the model number of your Bosch washer?
In that case it I would focus on the motor, since it would suggest the motor is running but isnt providing sufficient power to rotate a fully loaded drum, which may be due to failed windings or reduced current served by the PCB.
Have you placed a multi meter on the motor with the back off to see if your getting voltage across the spade terminals when the drum has clothes in it?
The sensor you are referring to regulates the speed of the motor, if this was faulty you would normally be presented with an error code on the display.
If the engineer checks your getting voltage to the motor this rules out the sensor as well, the video here gives a detailed list of diagnostics you can try.
However based on what your symptoms are, if your getting voltage to the motor but its not turning its a faulty motor, if your not getting voltage then it may be the sensor (but highly unlikely since you wouldnt expect it to work when there isnt any load in the drum), and I would be looking at the PCB.
Please let me know if you need anything else clarified.
No problem, appreciate if you can rate the service received today.
Should they (or you) have any further queries after carrying out the fault finding steps please feel free to ask