I will comment with a few warnings. Flattened primers are NOT how to gauge chamber pressure. You can blow up a gun
well before you see evidence of "flattened primers." The best second level indicator of pressure is measured velocity - which should track with the published data. If you are getting significantly higher velocity with a prescribed amount of powder, something is WRONG and your pressure is way above where it should be.
Felt recoil is a dangerous way to evaluate pressure. I can tell you a Trail Boss load in my 45-70 with a 480gr bullet that you can shoot easily and convince yourself it is a mouse-fart load. In fact, that load will probably blow a trapdoor to smithereens. It doesn't kick very hard in the Sharps, primers are unremarkable, and report is mild too.
Propellant type to case volume is carefully evaluated by manufacturers. I would be very careful reducing loads below those listed in the manuals, especially with large volume cases.
I would suggest downloading, reading, and digesting
From Ingot to Target: A Cast Bullet Guide for Handgunners and coming at the loading of cast bullets from the alloy end. Your alloy and expected velocity will then steer your propellant choice. Either way, have fun with your new found use of lead bullets!