DC-3 Mixture always on auto-lean / fuel flow / levers inop

If the Mixture lever is not pushed all the way forward, the motors do not start, which corresponds to reality. An “auto rich” function is not useful for starting, or is it?

I have a problem with the assignment of the mixture levers to the available controls. I have assigned the propeller speed control to the only free analog slider which my throttle unit has. As consequence I don’t have a free analog slider anymore and I have to assign the full-forward position of the mixture levers to a key combination. But none of the keyboard assignment options in the mixture section of the control options work. Who has an idea?