Hi again,
I really appreciate all the replies and suggestions! During the weekend I had time to do some experiments.
I wanted to keep the original circuit (because it was already assembled), if possible. I had the possibility of
separating field coil negative from dynamo negative.
Moved the Q2 transistor to the low side and, as suggested, it improved the voltage drop significantly.
The voltage drop is now around 1 Volt (@6V, 1A). I think this will be good enough for the time being.
Changing resistor R3 to a lower value did not make any significant change in the voltage drop.
Will probably keep it at 150 Ohm. After tweaking trimmer R1 the circuit gives a very nice cutoff at
around 6.9V, to protect the battery.
Need to wire everything up on the motorcycle and test it.
Thanks for all your help