As someone who used to sell In Car Entertainment (the other ICE) solutions and worked in Sound Systems one thing you will NEVER get in a car is audiophile quality sound. With such a mixture of hard and soft surfaces at different angles, speakers in the wrong place for an effective sound stage and amplifiers heavily constrained by the available voltages the task is fruitless.
It may be possible to improve what you have but good audio is nigh on impossible. You would be better off with a decent pair of headphones except that you could be charged with an offence if you did.
I have done upgrades on BMWs, Jaguars and Range Rovers and although it was possible to get some improvement with different head units what was quite common at that level was the factory fit speakers were fairly optimal and didn't need upgrading. At lower levels in many cases it wasn't so much the speakers themselves but more often how they were mounted or what was behind them. Fixed to overly flexible door cards with plastic sheet water barrier touching the magnet was never going to sound nice. BMW would mount rear speakers in the back parcel shelf of their saloons, fixed to the metal which was better but they would either have the entire boot as a cabinet or a second metal skin to make the box. It then didn't help with the speaker driving up into the rear window.
Finally what you may find is a speaker change to something better actually sounds worse. With modern electronics designers can "tune" the amplifier output base response to compensate for deficiencies in the sound. They can clip the top end if it sounds too tinny and lift the bass if needed.
