How to make a strong mug of filter coffee without any Coffee machine?
Chicory is completely a matter of personal taste.  

Trying to make a coffee concentrate and then dilute it with milk is a very unusual practice (at least without an espresso machine).  I don't know if the water will become saturated, but you might try brewing in a much larger volume of water, and then adding cream after which is a more typical practice.   

6 minutes is a little long for most peoples' tastes; 4 minutes is more typical.  You may be getting a more bitter cup than necessary due to overbrewing.

6 ml to 260 ml total beverage is very weak; you may try closer to 30 ml to 200 ml as a more typical starting ratio, which you can then adjust according to your taste.

Other than that, make sure your water is at appropriate brewing temperature (without a thermometer, bring it to a boil, then turn it off for a few seconds).  Don't use metal glasses, which lose heat  quickly; use something glass or ceramic.

See also:  Coffee Geek discussion on rations (in metric)