Elmer Keith went from 20:1 to 16:1 lead:tin in developing the .44 Magnum, so you seem to have rediscovered what he considered the best mix.
I was remembering wrong: the silver in casting alloy wasn't for hardness, but for better mold fill. I think Veral Smith was talking about an amount so small he was just adding a few inches of silver-bearing soft solder to a 20 lb pot. Must reduce surface tension. Laser Cast bullets have some silver in them according to
their site literature.
The Lyman Handbook has shrinkage for some alloys. No matter what the constituents do by themselves, once in an alloy their effect can differ. I also remembered Lyman's wheel weight numbers wrong by 1/2 a percent. They list it as 95.5:4.0:0.5 lead:antimony:tin. So, 4.0% antimony. The LASC site had something on the changes in wheel weight alloys over time, which apparently are regional. Mostly they are cast using recovered scrap as a primary constituent, so purity and consistency was never part of the picture unless you purchased virgin alloy.
Based in the above, Lyman shows the as-cast diameters for a 0.458" mold of theirs would be:
0.4575" lead
0.4583" wheelweights
0.4590" Lyman #2 alloy (90:5:5 lead:antimony:tin).
So, at least in the alloy, the antimony is not causing a shrinking problem, but rather seems to do the opposite. The WW and #2 alloy melting points are lower than for pure lead, so the mold itself would be slightly smaller, yet the bullets come out bigger.
As to copper, I see no problem with it except maybe for hollow points. The original Babbitt (named after its inventor, Isaac Babbitt, I learned; hence the capitalization) formulation apparently had copper in it to make it harder and more durable than lead and tin and antimony alone, though nobody knows the exact original formula. Now there are a number of both lead and tin based Babbitts, but they all cast well and precisely to make journal bearings. A fellow on another board got a big supply of scrap Babbitt at one point that he said made wonderful non-expanding wide meplat bullets that survived impact well, but I don't know what temperatures he was shooting in or how heavy the bones he hit were. Given that Babbitt has seen use in engine journals, I don't expect it is too brittle in the cold. Have to be tried, though.