Muzzle flash is not a bad thing at all, all good loads have it.
You are still burning right till the end of the barrel, and probably getting max velocity, with a longer, slower push. Don't sweat it.
Going with a faster powder will give you higher pressures, and less velocity.
My 257 Roberts Ackley Improved uses 51gr of powder, crushed into that tiny case.
Shoots darts with a 75gr bullet, and I am pushing almost 4000 FPS
Talk about a fireball!! It blows a 5' flame, and lays the grass down for 5 yards in front of it