Can't you use the USB-C output of the phone to a DAC? Do you not have any digital input to your 'vintage receiver'?
If not, you can indeed use a 3.5mm (1/8") analog to RCA cord. They're cheap.
I did use that from my PC direct to an analog RCA input on my new Luxman L-505uX Mark 2 integrated amp. Of course I can't do any correction without adding a DSP into the mix, but it still enabled me to adjust my subwoofer and see the differences. Now I have a separate DAC and could input to that.