With the new strategy the population settled at about 9100. There may be various reasons for this but in the end it was not the ideal solution. I have gone back to the original layout and improved the railway connections for the cargo in the heart of the city, allowing cargo trains to leave/enter the station at higher speeds, thus making the time for cargo to travel shorter.
The end of month delays are now about 1-2 seconds, so hardly noticeable. The population is now 11850.