Goodsteel:
Two things I would do is to oven heat treat or at least water drop when casting and also fill the hollow base with hot melt glue.
One other comment ~ and I can't absolutely confirm this but I think that big flat nose might be too much of a good thing.
I tried full wadcutter hollowbase slugs (more like a flat nose Foster than your pellet design) and they did not give very good accuracy at 50 yards where simply changing the nose to TC style with a large meplat but also the conical bevel seemed to improve accuracy noticeably. I fiigured the big wadcutter nose disrupted too much air flow around the slug so it wasn't stable. Whatever, they didn't work well for me.
Also, my understanding of the Lyman sabot slug with the big flat nose is that they become unstable when dropping to transonic velocity and can tumble when shot from smoothbore. Okay when shot from rifled bore but unstable without the spin.
Something you might try is to make a simple form out of CPVC pipe or pipe coupling (or metal if you want) so that you can slide the slug in leaving a 1/2" or a little more empty "tube" behind the slug then fill the skirt and form with hot melt glue. You will need to use some grease or something as a release and since yours is bore size you might want to add a one wrap paper tube to cover the glue. This gives you an attached wad slug with longer, lighter "skirt" and support for the skirt as well (I see in the photos that the skirt has squished up a bit). The longer "skirt" will provide some more drag which may help. It is easy to try anyway.
I have been doing this with home made hollowbase slugs sized to fit into a shotcup and it seems to be working well. Call it a home made "poor man's" Brenneke or AQ slug. Simple to make and they are giving me pretty good accuracy so far though there is testing yet to do.
Longbow