Hello,
Is there a way to set external storage as defaut storage for the camera ?

For now i installed caméra mx to do it. But i didn't manage to do it with native camera.

Native camera app is very limited. I highly suggest to install the opencamera app or other camera apps that will let you do what you want

With the native camera app, you can only do it setting and formatting your SD card as Internal Memory (same option to install apps to the sd storage). Of course like so you cannot use your sd card out of the phone.