How to eliminate stringing and regulate flow rate?
Stringing can be fought by increasing the retraction length, but 6.5 mm (the default in Ultimaker Cura) is already pretty much. Please do note that too much retraction can lead to sucking air into the nozzle, which can lead to air pockets/holes.

Basically there are 3 mechanisms that are in play:


Retraction
Temperature
Speed


You have tried changing multiple parameters already. You may have better luck in reducing the temperature of the nozzle even further, start with a decrease of 5 &deg;C. All that is left to do is increasing the travel speed.

There are many stringing test prints to print to fine tune stringing.