Stringing can be fought by increasing the retraction length, but 6.5 mm (the default in Ultimaker Cura) is already pretty much.
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 °C. All that is left to do is increasing the travel speed.
There are many stringing test prints to print to fine tune stringing.