BP 2F with a very slight charge (5 grs) of Unique just above the primer-with rest of case in BP burns very clean. This is a duplex loading and is widely used as a 10% SMKL/BP -particularly in strong actions like the Sharps. Newer SS rifles in 45-70 are warranted for smokeless powder and black-but one has to be both conservative and careful.

I have an 1885 Winchester HiWall SS that in 45-60 that gets special treatment. Made in 1887.

I use ten percent 5744 with Goex cartridge in various BP cartridge rifles for many years. As you say, burns perfectly clean. It doesn't raise pressure for me.
I used to use I just forgot the number; it was discontinued years ago but used to be the go to for duplex loads.

Du Pont 4759