The main step I went thru was to remove C601/602/603/604 and adjust and solder a 8 pins 1.27 male smd header (2x4) in order to have something clean, easier to prototype in, and, in case I want to go back to the initial setting : plugging a 1.27 female header on it, with 4 caps soldered on the header.

I implemented on a breadboard the missing Symmetric Low Pass filter for the right channel. Surprisingly, even with a basic LM324, the result is very satisfying, no major difference with the existing left audio from a noise point of view.

Breadboard_Mono_Right_Small