I'm glad to let you know that I finaly manage to fix it few hours ago, even If I dont really know how I did that...
I've made a bunch of cleanup, reinstalled/uninstalled stuff which might have been badly uninstalled by the past like Afterburner, Gaming App, Control Center, etc...
It finalement worked with old 18.5.1 Adrenalin and now with latest drivers, I'm ocing/ucing with OverdriveNtool.
I guess there might have been somewhere an old config' file or script of an unidentified software that locked everything else.
The only odd thing is that I cannot set target temp. below what bios says and GPU fans can't go below 30% , but I can oc/un any other value and mining ops look stable.
Thanks anyway for your support