I think staying with shotgun-type powders might be a good idea - to keep the speeds in the lower range, but then, perhaps that is why it is not grouping - just casting a suggestion. I would try a slower burning powder, rather than the trap-type powders.

The .729" sizer might be the ticket.

I would try Herco, SR7625 and SR4756. All three of those have worked well for me in a smooth 12 bore.

I did buy some Longshot and Steel for testing, but winter got in the way of that - another 8 to 10" of it last night.

Lube is another avenue for group effect. They will make a difference at times.