Everybody has given you good advice so far.
Forget about bullet hardness, it can be deceiving. Think about bullet/alloy STRENGTH. The strength of the bullet metal to take deformation of the pressure of the load, and not deform. That's the second part of the equation, pressure. Pressure causes velocity, so those terms can be used to say the same thing.
Richard Lee has a good write up in his loading manual about the pressure equation as it refers to alloy strength. Another good source is the write-up at lasc here;
Oh, you said your lead free solder was 5% tin, it SHOULD be 95% tin--5% antimony. It is a great source of tin, with the added help of antimony. Be careful to not get the lead free solder that has copper in it.
Now, wheelweights already have some tin in them, so adding too much solder can add too much tin. It won't cause trouble, BUT it sure adds cost! A pound of lead free solder will add 1% tin to 100 pounds of pure lead. If added at that ratio to wheelweights, you would be close to 2% where you want to be. You can figure ratios for smaller amounts of lead, Like a certain length of lead free solder for a pot of 20 pounds of lead.