Black powder, of course, is also an option.
If you look at Lyman's data in the 12 bore section, you will see various powders and wad columns with a variety of projectiles - styles and weights.
As far as I know, there are no slugs in the 750gr. range, however that is less than 1 3/4oz., being 1.71ounces.
There are shot charges for 12 bores listed in the shot section and are shot with Unique, Herco, Blue dot, Steel and Longshot powders.
taofladermaus - a U-tube channel, had shot many 12 bore loads with slug weights in this range & heavier using Longshot powder. Often his loads run 30gr. and over.
I would peruse Lyman's shotshell loading manual. Typically, wad columns of cork, fiber or donnaconna produce much less pressure than-do the plastic wads of today. Slug loads listed in the manuals run both.