Finally I made a total new installation with Mojave and with Imac pro system def.
I made a mix of these :
https://www.tonymacx86.com/threads/...ojave-on-any-supported-intel-based-pc.259381/
https://www.tonymacx86.com/threads/...ojave-successful-build-extended-guide.255082/
It was very painful and I got a lot of problems to fix but now it's working great… for the moment (3 weeks only).
I don't know if problem was fixed by Mojave or by the Imac pro def.
I guess Logic works better with Imac pro def…
I can't remember what I took in each tutorial.
I just remember I made the USB Mojave installer by following the usual guide and I took the KGP EFI folder from the imac-pro tutorial. Many thanks to KGP by the way…
Sorry if I do not have a better "scientific" explanation. I'm just happy to be out of this nightmare.