Sounds like you want to "make bullets", but, have no intentions of hitting the same thing twice. The whole slug / wad thing is designed mostly for rifled barrels. Punching a wad out of a smoothbore is not going to give much in accuracy over distance.
I cast my own .58 Minie bullet and reload them in the same similar combination as you stated for 20 gauge, but, I only use my reloads in my rifled slug gun. I need to place a consistent shot on a Deer, a smoothbore is not likely to do that. When that slug/wad leaves the
barrel, it could go anywhere. This is what my cast and reload looks like in 20 gauge. The filler wad in this photo is no longer used now. This is my answer to the more expensive Remington BuckHammer.
I use 26 grains of Hodgdon HS-6 for my charge with a 2-3/4" hull. BPI Commander wad, and I fold crimp as well. The filler / card is of no requirement here. My reloads and recoil feel the same as compared to the Remington BuckHammer 20 ga. slugs. Practically spot-on.
Also pictured below is a sample of the 4 different hulls I prefer to reload with. Very sturdy hulls and I can reload them many times each. The Remington on the Left and the Federal on the right are super strong hulls. I prefer the Federal over the Remington because it is a high-brass base. The Winchester is "ok", but, not my top choice. I will also tell you this. If you order new/primed hulls, be sure to measure the hull. The new hulls I ordered from BPI were 1/4" too tall when they are marked 2-3/4" you still have to trim them down.