I see a lot of people complaining that the Fairphone 6 doesn’t have an Aux jack.
Just use an adapter cable.
A 3.5mm Aux jack takes up a significant amount of space just to connect a few wires that could be connected through USB-C anyway, that space could be used for a bigger battery.
Even if there was a good enough reason to keep Aux it should be 2.5mm Aux and not the usual 3.5 as it does exactly the same thing but uses less space
There are “gaming” phones that have two USB-C ports for the purpose of not obstructing the hands while gaming, but they can also be used as you describe (iirc). It would be a nice feature on mainstream flagships.