Originally Posted by Mr XY
Thank you so much for this!! amazing!
If i could ask one more thing - does this seem like a good chice for the switch? A402S1RNCBE C&K Components | Mouser
Or if you could please recommend me what you would use?
That looks like an excellent choice. The large silver contacts will have low contact resistance and it's a "non-shorting" action which will keep the L & R channels from being momentarily connected together when the switch is rotated (although that's really not an issue for line-level" signals). A pair of those will work fine.
When working with a "new" multi-pole switch for the first time, it's often a good idea to confirm the connections with an ohmmeter. Check each of the poles for the connections in Pos -1 and then physically change the switch to Pos-2 and re-check that the other poles are actually connected as you expected.
A lighter duty switch would also work fine - Mouser has some Push-Button slide action 4PDT units that will work, but if I were building it, I'd use the rotary switches so there is no doubt about which source, and which stereo orientation is selected.
Good Luck with your project!