Creating a custom (car) Radio

Introduction

I've got a luxury problem, I own 2 old cars, one has no radio at all and on the other one my cassette drive based radio died and I replaced it with a modern one, but to be honest, it sucks. My main issue with it, I can't have multiple inputs at the same time.
For example, if I use my mobile phone connected via Bluetooth for navigation, I can't play from the FM oder DAB+ source at the same time. So either music or navigation directions, not both at the same time.

Also, it's one of those modern bling bling things which does not match the design of the car.

So, let's build or own.

For now I'm thinking about these features:

• FM radio
• Bluetooth connection for hands free telephone functions using the car speakers and a microphone
• a small OLED screen for various display things (RDS, connection indication, radio station)
• aux input via 3,5mm input for external non-Bluetooth devices
• individual mixing of those input with automatic support for muting some sources (like radio when there is an incoming phone call)
• DAB+ would be nice

For the various inputs, there are existing chips on breakout boards, except for DAB+, I wasn't able to find a module for this.
Also the mixing might turn out more complicated, but let's start with the easy parts.

As a controller I'll start with an Arduino Nano, if it needs to have more oompf (yes, the correct technical term), I can upgrade at any time.
For the screen, I'll mount an 0,91 inch 128x32 OLED black and white display where the old display was mounted, if I'm fancy, maybe 2 ;)
I'll try to reuse as many buttons from the original radio, so it looks almost stock.
The radio part is a Si4703 Radio FM RDS RBDS Tuner Breakout Board that can be controlled via I2C.
Bluetooth connection as well as the hand-free phone function will be done with a F-6188 Bluetooth module.
I'm not sure about the audio mixing, probably just a simple op-amp circuit, muting of the FM-Radio via software, AUX muting maybe via relay.
Time will be kept with some I2C real time clock module. Don't have decided which one, yet.
As an output driver for the speakers I'll have a look on off-the-shelf cheap amplifiers.
For now, no fancy 4096 channel spectral analyzer or a 10 inch touch screen.
Maybe I'll reuse the existing bass/treble adjusters, but I'd need to find a suitable schematic for it. It will be done last.

So in total, nothing extreme fancy, but tailored to my needs.