Shoot several powders at different charge levels and bullet combinations to find out what the gun likes. There's no one-size-fits-all when it comes down to this aspect with any gun. Since you need to use caps and not primers you'll need to stay away from the BH209 powder since a hot primer is needed to reliably ignite it. There is a whole world of different black powders along with Pyrodex and T7 as granular subs. All blacks and Pyrodex will fire easy with the #11 caps. T7 may need a magnum, cap in cooler areas of your state.
If it were me I'd skirt the pyrodex and go right to the T7 to see how it does with different loads and bullets using magnum caps or the musket caps. T7 is the most powerful of the blacks and subs. If T7 doesn't cut it for some reason I'd look at the higher end blacks and start over with load-bullets combos that are accurate. If you're going to shoot sabots, start the T7 at about 70 grains by volume with bullets in the 250 to 300 grain range and work the load level up by ten volume grains for each trial. With the same bullet weights and black powder I'd start at maybe 75 grain by volume. Plan to run a damp patch down the barrel after each shot with any of these powders.