Decided to place the control board and the audio sub-system on new boards. This way will have a more standard fitting by means of header pins.
In the mean time I added a tone decoder (LM567), and sound card isolation on the audio board. On the control board I added the code for PTT and redesigned the input pins for the band and mode buttons.
I will for sure use almost, if not all, of the Arduino Uno imputs and outputs.
Here's the audio part (top) and the control board (bottom) with the PTT button for testing:
And the underside of the control board with the Uno:
Have a nice week!