BMW satnav does what it says on the tin. It works effectively but doesn’t have the bells and whistles of TomTom although it does show traffic updates and shows congestion etc. However, who chose the colour scheme needs a lesson in ergonomics and human interfaces. Grey and green are not easily distinguishable.
The display though is the interface for all car functions though.
If you go third party satnav (your only choice I suspect) is how the audio directions are played. If you want to use the car audio, that means you will have to use the same device for your music etc. The car audio won’t mix an external input with its radio/HD etc.
If you have bmw connected services (and subscribe) you can upload routes. Most phone based satnav systems you can’t.
Retrofitted bmw satnav, if it can be done, isn’t going to be cheap. It’s not just the satnav unit but also GPS receivers (at the top of door pillars I believe). Also, you have coding the car to recognise it is there. Many forum posts indicate that BMW won't do it, but, I'm sure someone will.
I thought it did tell you which lane to take though but only on the display.
Cartronics do a Z4 upgrade.
https://www.cartronics.co.uk/BMW-Z4-Navigation-Upgrade but it looks like it is a fixed add-on.
Your solution will really depend how OEM you want it to be and how much integration.
I'd be tempted to speak to one of the many car systems integrators - most of these that do audio also do satnav.