I've shot flintlocks for almost 30 years...I usually prime with FFFg...
It shouldn't be much slower than FFFF...In hunting situations I prefer the courser grade because it doesn't absorb moisture as quickly....You probably are having slow ignition problems due to a touch hole being too low or not coned from the inside...If you don't want to take the time to properly fix the ignition problem then yes, you can grind FFFg powder to a finer grade...but remember Pyrodex has a much higher ignition temperature than real black powder...It came out in the '70s and was developed so it could be shipped UPS.

Flintlocks were made to use real black powder....Not 777 or Pyrodex...