I just came back home, jumped on my arduino, edited the send_command_to_serial(char command[50]) function and replaced
strcpy(synchronized_buffer_send.buffer,command);
by
- ```
memcpy(synchronized_buffer_send.buffer,command,50);
And it worked ! ! ! Thank you so much for your help 
! ! ! !
Jimmus:
Yeah, strcpy is not the right function to use here. It will stop at the first 0 byte.Really, though, doesn't EasyTransfer do all this for you? Wouldn't it be easier and way more efficient to just use your structures directly?
This is what i tried at first. But for a reason I may not fully understand yet, I only got the Easytransfer library to work with one send structure and one receive structure per device.
I tried using multiple structure by assigning one send and one receive on Serial1, another 2 structs on Serial2 but the result was a complete mess with the value of one struct overwritting the value of others on the received buffer... I tried with small structure of less than 20 bytes... I also tried 125 bytes structure (containing all structure) but the error rate was too high with only 1 successful transfer every few minutes ...
I used a simplified sketch for test purpose and only managed to get it work with 2 structs of less than 50 bytes for a 100% success rate other 8 hours of test. This is why I tried to "workaround" the problem. And even now, I don't know if the limits comes from the library or because i'm not using it correctly, so I'll make sure to figure that out later
Once again thanks for you help guys !