<p>A few minutes after finishing a print job, the filament is solidified in the nozzle and the nozzle-throat. When I start another print job a while later, the filament is not sufficiently melted and the nozzle is obstructed. Do I need to clean the nozzle after every print job ? or is there a practical method to overcome this difficulty ?</p>

Your solution will not cool all sides effectively. Firstly don't use zip ties; get thermal tape.
(
https://www.amazon.com/Thermal-Interface-Products-Heat-Sink/dp/B00QSHPH8E/
Secondly, the heat will need to travel around the outside of the motor to get from the side that doesn't have the water block. Its expensive but you could use
Pyrolytic Graphite Sheets
to wrap around the outside of the motor, to get the heat to the water block faster.