Hi @tony-white
In the end, I tried and tried again, with a new raspberry, a new pixhawk, a new power module, a new sd, and add a usb-c power supply directly to raspberry.
As soon as I had a system that worked, I switched one by one with the old components, starting by running on the new sd and old raspberry, then I tested the old power module, then I tested the old pixhawk.
what’s strange is that when I do this, everything works again, and I have telemetry feedback in blueos 1.3.1.
The only problem I have is with the old pixhawk, the telemetry is frozen and doesn’t work.
I don’t know how to identify the source of the problem, but I have been able to identify that I have a bootloader problem.
I QGC, in vehicucle firmware page, Ihave :
Version: 5
Board ID: 9
Flash size: 1032192
I’ve try this method from ardupilot forum page :

joy123Joydeep Bhowmik
2
Apr 2024
I was going through the discussions to update the bootloader to from 4 to 5. If you are using pixhawk with 2.4.8 which supports 2 mb flash but can’t upload v3 firmware due to bootloader version 4, here is what worked for me: Download any fmuv3 firmware with bootloader which must be less than 1mb if you are using fmuv2. I downloaded ardusub firmware ver 4.1.2 with bootloader. Open mission planner and select custom firmware. select board type fmuv3 and flash this firmware. Reboot and connect by mavlink. Go to install firmware tab and it will now show “update bootloader” click at and bootloader will be upgraded. You can now flash full firmware with all its functionalities.
But I’ve another Issue, I’ll investigate later on this one.
For the moment it’s functionning with the second pixhawk.
I’ve to open a new issue because I can’t use the h264 camera. I can see it with dmesg
but I not be able to see it. Maybe i should add a 5v power supply directly to the camera, not from the raspberry.
It’s very strange, an undervoltage issue can cause all of this quagmire ?