Lots of exotic ideas, But I'd just stick with a Lead alloy boolit, myself.
Alloys with high antimony content will more likely shatter on impact, which dilutes the energy.
an alloy with equal parts antimony and tin will be "tougher" and more likely to hold together than alloys that don't have equal proportions...which will deliver the most energy to the target, making the best chance to penetrate it.
Myself,
I'd cast some Lyman #2 alloy, then heat treat, you'll likely get near 30 BHN.
(I have heat treated 94-3-3 up to 24 BHN)
I'd use a heavy boolit, like the Lee 310gr RF and work up a load with H110.
Good Luck