Additional hardware on Pixhawk4 mini for a quadcopter-x

Hello everyone!
I have put a lot of thoughts on this and I am slightly lost now.
I am trying to connect additional pieces of hardware to the Pixhawk 4 mini.
Mainly some LED module and speakers.
At first I thought I could connect it on the GPIO located in the back of the board (AUX 5-6 or Main out 7-8)
Then I tried the CAP… and I even tried to connect a raspberry PI4 to the I2C/UART cable of the Pixhawk4 Mini. At that stage I cant think of a way that has (not) worked or which parameter to input in PX4 to get the job done.
Would it be easier with a companion computer that manage ROS packages and launch bids of code ?
Any help to get this straight would be highly appreciated
Many thanks