The ranges expected are in your favour as-is your current accuracy. When I worked with a SXS smoothbore I shortened to 26" bls - might have been 24" and mounted iron sights in the solid rib (along with scope bases) I used the cup wads cut from the base of 'spent' WAA12 white trap wads I picked up at the local trap range. These went on the powder, then the cards and fillers wads and ball took up the rest of the space to a folded crimp.

Whether folded crimp or roll crimp depends on how much material you have left for the crimp.
Lyman's Shotshell handbooks have a great deal of loading data for 12 bores - with the slug you are using as well as others and round balls too. Data to almost 1,600fps is readily available from these books.

Here, Ed Huble has an ongoing thread "12 Gauge from Hell" - with excellent load data and information if you want to improved your ballistics even further. I suggest it is best to start at the first page if you are unfamiliar with this thread.