Very nice mortar!
You need definitely to get your hands on some real black powder. GOEX can be purchased online and shipped to your door, in as little as five pound lots from PowderInc. That's not an insignificant up-front investment, about a hundred bucks. But it goes fast, and as ammunition prices go these days, it's really a bargain. If you can put together a larger order, the price per pound drops considerably since the HAZMAT is a flat fee up to 25 pounds.
And it's well worth it, trust me.
Pyrodex, as you've found, needs a good seal in the bore and therein lies potential safety issues. And it's nasty stuff, real BP is as natural as the ingredients (charcoal, sulphur, saltpeter) and cleans up easier than any of the subs. Pyrodex and other BP subs are great for hunting and target shooting with modern inline muzzleloaders. For muzzleloading artillery, not so much.
A fine mortar such as yours deserves nothing less than the real thing!
Once you've gone black you'll never go back...