[Project idea] Help with Volume control on my AMP

I have a Raspberry Pi 3B+ running Volumio 2.873, plugged into a Music Streamer 2 USB DAC that goes into my Technics A700 mk3.

I am wanting to control the volume of the amp directly from the raspberry pi, as I don’t get any volume control through volumio. I have done some research and at least one person has implemented an irblaster solution, but I was wondering about possibly controlling the volume via some sort of relay and tapping into the internal circuitry as I want to avoid an external mess of wires!

I’ve found a copy of the service manual online and there is a separate volume control PCB. So this idea doesn’t seem so far fetched. As shown in the circuit diagram below.

Could someone help me figure this out as I am not very knowledgable about electronics?

Also, I imagine there would be a bit of coding to enable the volume within the gui to output via the gpio pins and I would also need some help figuring that out at a later date.

Perhaps there is a more elegant solution…

Any thoughts and suggestions greatfully recieved.