Without the overpowder card, you have no gas seal, and when you might expect velocity of 1200 fps with a given powder charge, the actual velocity could be as low as 600 fps! You might as well put shot directly into the powder for all the pattern you will get. That patch will be burned up and through by the powder gases on the way out the barrel. Those gases will blow through the shot and distort them, and send them every which way as they leave the barrel. The gases will actually begin leaving the barrel before the shot gets there.
Other than that, you have a good idea!
Seriously, don't try to reinvent the wheel. Just about everything that can be done with a shotgun or smooth bore, has been done. Read the earlier posts on the forum, and you will get excellent instruction on how to load your gun. If you have reloaded modern shotgun shells, the only difference between them and a muzzle loading smoothbore is that the barrel becomes your cartridge. The order that things go into the barrel has to remain the same, and we pretty much do with paper components what manufacturers of modern loads do with plastics.